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Precondition

###Precondition "Several"Several thousand" means at least two thousand. Since they are only 200ly away, this means that their first radio signals (which were likely close to what we would consider "radio") are already history. What we receive now, if we receive anything, is 1700+ years ahead of our state of development.

The signal

###The signal OverOver two millenia of "radio availability" makes it likely enough that their signals are at the very least something like a forward error-corrected and compressed digital (or trigital, who said an alien bit can't have three states?) signal. Forward error correction would mean they could send a lot more energy-efficiently, which is good for them but not so good for you trying to receive their signals from far away, and compression would mean that whatever you receive, if you receive something, looks like noise.
They might as well use a form of "radio" that we are unfamiliar with (unusual high frequencies?) or entirely unable to interprete. Their form of radio might even be some form of quantum communication, which by design is only receivable by designated subscribers and makes "eavesdropping" from the outside impossible. Quantum communication is something we are presently almost getting to work with photons (well, saying "almost" is wrong... it works, it just isn't ready for mainstream yet). An advanced alien race might as well have some form of "quantum radio" with designated recipients, why not.

The Wow! Signal

###The Wow! Signal TheyThey actually tried to communicate (not with us, but maybe with a starship of theirs), and we actually did receive their message. Unluckily, we were unable to decipher or identify it as what it was, nor answer to it. Eventually, the signal was considered "just noise", or something a comet produced.

Reflection

###Reflection AnAn entirely different, more conservative, and plausible explanation would be ionospheric (or similar, whatever they have on their planet) reflection, in the same way as shortwave radio is reflected on our planet. This isn't even limited to the planet's atmosphere, their solar system could have an Oort cloud which contains some kind of particles that reflect 99% of all transmissions. Thus, they could even communicate with space ships within their solar system, or a colony on another planet, and we would still have trouble catching a glimpse.

Black matter

###Black matter BlackBlack matter might be another perfectly plausible reason why we didn't spot them. We know with rather good certitude that such a thing as black matter exists (though we don't really know what it is exactly, or how exactly it complies with our known laws of physics, but we are confident something is there). There might just happen to be some black matter that, for some reason, absorbs radio signals in between us and the aliens. Most radio signals broadcast quasi-light-like, which means if something is in between sender and receiver, the signal is partially or completely obstructed. That is the main reason why TV stations (or GSM stations) have their antennas on high towers or mountains -- their range is primarily limited by obstructing objects and finally Earth's curvature, not so much by the transmitter's power. It is also the reason why orbiting the moon is a no-fun experience for astronauts (there's no such thing as telling "Houston, we have a problem" with the Moon in between).

Technical limits

###Technical limits FinallyFinally, even without any special conditions, detecting another civilization which isn't actively trying to communicate is a daunting task, to say the least. Or, you could say, pretty hopeless.

###Precondition "Several thousand" means at least two thousand. Since they are only 200ly away, this means that their first radio signals (which were likely close to what we would consider "radio") are already history. What we receive now, if we receive anything, is 1700+ years ahead of our state of development.

###The signal Over two millenia of "radio availability" makes it likely enough that their signals are at the very least something like a forward error-corrected and compressed digital (or trigital, who said an alien bit can't have three states?) signal. Forward error correction would mean they could send a lot more energy-efficiently, which is good for them but not so good for you trying to receive their signals from far away, and compression would mean that whatever you receive, if you receive something, looks like noise.
They might as well use a form of "radio" that we are unfamiliar with (unusual high frequencies?) or entirely unable to interprete. Their form of radio might even be some form of quantum communication, which by design is only receivable by designated subscribers and makes "eavesdropping" from the outside impossible. Quantum communication is something we are presently almost getting to work with photons (well, saying "almost" is wrong... it works, it just isn't ready for mainstream yet). An advanced alien race might as well have some form of "quantum radio" with designated recipients, why not.

###The Wow! Signal They actually tried to communicate (not with us, but maybe with a starship of theirs), and we actually did receive their message. Unluckily, we were unable to decipher or identify it as what it was, nor answer to it. Eventually, the signal was considered "just noise", or something a comet produced.

###Reflection An entirely different, more conservative, and plausible explanation would be ionospheric (or similar, whatever they have on their planet) reflection, in the same way as shortwave radio is reflected on our planet. This isn't even limited to the planet's atmosphere, their solar system could have an Oort cloud which contains some kind of particles that reflect 99% of all transmissions. Thus, they could even communicate with space ships within their solar system, or a colony on another planet, and we would still have trouble catching a glimpse.

###Black matter Black matter might be another perfectly plausible reason why we didn't spot them. We know with rather good certitude that such a thing as black matter exists (though we don't really know what it is exactly, or how exactly it complies with our known laws of physics, but we are confident something is there). There might just happen to be some black matter that, for some reason, absorbs radio signals in between us and the aliens. Most radio signals broadcast quasi-light-like, which means if something is in between sender and receiver, the signal is partially or completely obstructed. That is the main reason why TV stations (or GSM stations) have their antennas on high towers or mountains -- their range is primarily limited by obstructing objects and finally Earth's curvature, not so much by the transmitter's power. It is also the reason why orbiting the moon is a no-fun experience for astronauts (there's no such thing as telling "Houston, we have a problem" with the Moon in between).

###Technical limits Finally, even without any special conditions, detecting another civilization which isn't actively trying to communicate is a daunting task, to say the least. Or, you could say, pretty hopeless.

Precondition

"Several thousand" means at least two thousand. Since they are only 200ly away, this means that their first radio signals (which were likely close to what we would consider "radio") are already history. What we receive now, if we receive anything, is 1700+ years ahead of our state of development.

The signal

Over two millenia of "radio availability" makes it likely enough that their signals are at the very least something like a forward error-corrected and compressed digital (or trigital, who said an alien bit can't have three states?) signal. Forward error correction would mean they could send a lot more energy-efficiently, which is good for them but not so good for you trying to receive their signals from far away, and compression would mean that whatever you receive, if you receive something, looks like noise.
They might as well use a form of "radio" that we are unfamiliar with (unusual high frequencies?) or entirely unable to interprete. Their form of radio might even be some form of quantum communication, which by design is only receivable by designated subscribers and makes "eavesdropping" from the outside impossible. Quantum communication is something we are presently almost getting to work with photons (well, saying "almost" is wrong... it works, it just isn't ready for mainstream yet). An advanced alien race might as well have some form of "quantum radio" with designated recipients, why not.

The Wow! Signal

They actually tried to communicate (not with us, but maybe with a starship of theirs), and we actually did receive their message. Unluckily, we were unable to decipher or identify it as what it was, nor answer to it. Eventually, the signal was considered "just noise", or something a comet produced.

Reflection

An entirely different, more conservative, and plausible explanation would be ionospheric (or similar, whatever they have on their planet) reflection, in the same way as shortwave radio is reflected on our planet. This isn't even limited to the planet's atmosphere, their solar system could have an Oort cloud which contains some kind of particles that reflect 99% of all transmissions. Thus, they could even communicate with space ships within their solar system, or a colony on another planet, and we would still have trouble catching a glimpse.

Black matter

Black matter might be another perfectly plausible reason why we didn't spot them. We know with rather good certitude that such a thing as black matter exists (though we don't really know what it is exactly, or how exactly it complies with our known laws of physics, but we are confident something is there). There might just happen to be some black matter that, for some reason, absorbs radio signals in between us and the aliens. Most radio signals broadcast quasi-light-like, which means if something is in between sender and receiver, the signal is partially or completely obstructed. That is the main reason why TV stations (or GSM stations) have their antennas on high towers or mountains -- their range is primarily limited by obstructing objects and finally Earth's curvature, not so much by the transmitter's power. It is also the reason why orbiting the moon is a no-fun experience for astronauts (there's no such thing as telling "Houston, we have a problem" with the Moon in between).

Technical limits

Finally, even without any special conditions, detecting another civilization which isn't actively trying to communicate is a daunting task, to say the least. Or, you could say, pretty hopeless.

Notice added Hard Science by HDE 226868♦
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Damon
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###Precondition "Several thousand" means at least two thousand. Since they are only 200ly away, this means that their first radio signals (which were likely close to what we would consider "radio") are already history. What we receive now, if we receive anything, is 1700+ years ahead of our state of development.

###The signal Over two millenia of "radio availability" makes it likely enough that their signals are at the very least something like a forward error-corrected and compressed digital (or trigital, who said an alien bit can't have three states?) signal. Forward error correction would mean they could send a lot more energy-efficiently, which is good for them but not so good for you trying to receive their signals from far away, and compression would mean that whatever you receive, if you receive something, looks like noise.
They might as well use a form of "radio" that we are unfamiliar with (unusual high frequencies?) or entirely unable to interprete. Their form of radio might even be some form of quantum communication, which by design is only receivable by designated subscribers and makes "eavesdropping" from the outside impossible. Quantum communication is something we are presently almost getting to work with photons (well, saying "almost" is wrong... it works, it just isn't ready for mainstream yet). An advanced alien race might as well have some form of "quantum radio" with designated recipients, why not.

###The Wow! Signal They actually tried to communicate (not with us, but maybe with a starship of theirs), and we actually did receive their message. Unluckily, we were unable to decipher or identify it as what it was, nor answer to it. Eventually, the signal was considered "just noise", or something a comet produced.

###Reflection An entirely different, more conservative, and plausible explanation would be ionospheric (or similar, whatever they have on their planet) reflection, in the same way as shortwave radio is reflected on our planet. This isn't even limited to the planet's atmosphere, their solar system could have an Oort cloud which contains some kind of particles that reflect 99% of all transmissions. Thus, they could even communicate with space ships within their solar system, or a colony on another planet, and we would still have trouble catching a glimpse.

###Black matter Black matter might be another perfectly plausible reason why we didn't spot them. We know with rather good certitude that such a thing as black matter exists (though we don't really know what it is exactly, or how exactly it complies with our known laws of physics, but we are confident something is there). There might just happen to be some black matter that, for some reason, absorbs radio signals in between us and the aliens. Most radio signals broadcast quasi-light-like, which means if something is in between sender and receiver, the signal is partially or completely obstructed. That is the main reason why TV stations (or GSM stations) have their antennas on high towers or mountains -- their range is primarily limited by obstructing objects and finally Earth's curvature, not so much by the transmitter's power. It is also the reason why orbiting the moon is a no-fun experience for astronauts (there's no such thing as telling "Houston, we have a problem" with the Moon in between).

###Technical limits Finally, even without any special conditions, detecting another civilization which isn't actively trying to communicate is a daunting task, to say the least. Or, you could say, pretty hopeless.

There's the thing about arc length. For something hundreds of light years away, you have to aim darn precisely, and you had better have a really, really directional antenna. But this is not the biggest problem.

The transmitter of a non-trivial nationwide TV station, which is among the strongest "normal" radio emitters that we have (about 2,000 times stronger than e.g. a typical GSM base) typically has EIRPs in the 100kW range. "Kilowatt" sounds like a lot, but it is really a quite small number.

The radiant flux density is EIRP/4π*d2

Note the innocent little "2", and note that d is actually 1018 meters. That's... a mindboggingly huge number, especially after you square it.

Anything "kilo" is too ridiculously close to zero for being measured if you divide it by 1036. Thus, catching any kind of "normal" signal that isn't deliberately directed at us with an extra super strong designed-for-interstellar-hello transmitter is pretty much hopeless.
Certainly, one can more or less arbitrarily scale up the size of the receiving antenna array. Naively, if you don't get enough Watts per square meter out, you just throw in more square meters, and there you go. But the problem is that you also catch more noise. Some of this noise is many orders of magnitude larger than the signal. The chance of actually catching something meaningful is rather dim.

Plus, there's quite a few stars out there. Neglecting everything else, and assuming that we are perfectly able to receive radio from any planet up to 200ly away, there's only about a 1:100,000 chance that we randomly pick the correct star system. Trying them all is possible, of course, but it takes several minutes to rearrange an antenna array, so this task would take anywhere from years to decades.
It might just happen that we only found them now because it took us so long to pick the right star.