It's all about being in the know
Even today, oceanic salvage isn't about detecting the existence of a derelict ship. It's about knowing that a ship didn't make port. Why?
Because the ocean is big. Really, really, really big, and when ships crash... they tend to sink, making them really hard to detect.
Space is, of course, really, really, really big. Big in a way that makes finding something in our ocean a lot more like finding your left sock in your bedroom. Worse (and this is important), unless you have something like faster-than-light transportation and/or communication, nothing about the crash will travel faster than the speed of light. In other words, if the derelict lost one light-year from any inhabited planet had a transponder (and if it were working), you wouldn't know where the ship was (in the worst case) until years later when the signal had arrived at not one, but two populated planets (triangulation...).
The idea of using something like radar (using any wavelength along the EM spectrum) is even worse. If you had enough power to pump the signal out constantly (say, one massive, cook-everyone-in-the-system pulse daily), you would be at least one year waiting to detect the ship, and at worst two (remember, the example derelict is 1 LY away).
That's a lot of wishful thinking
On the other hand, if you knew every trade ship's schedule and knew when one didn't make port as scheduled, you'd not only know there was an opportunity, you'd also have a better-than-average idea of where to look.
So, as usual, knowledge is king. And knowing where things should have been is half the battle when it comes to knowing where they are.
But, just as a frame challenge, would it be practical to have interstellar salvage?
I've been working on the assumption that salvage could occur anywhere. That's actually a really bad assumption. Space isn't just really, really big. It's unimaginably humongous. Without some really convenient things (like a continuously operating transponder and generations worth of time), the time to get there plus the cost to get there and back would never be worth the effort. And that assumes you can find it once you get there. If you had to travel a year to get to the last guaranteed-known location, that ship will have moved, possibly a lot, before you could arrive.
But what if we make some practical assumptions?
No matter what Hollywood things, even if you have FTL, pirates won't operate in deep space. Even the idea of a shipping lane is a volume of space so ridiculously large that by the time pirates detected a victim's passing, they couldn't get to it before it was out of their control. Besides, there's no support in the middle of the void, and piracy needs support (at least someone to fence the goods to!). That suggests pirates will act at known stopping point like gas giants used for refueling or within star systems that require ships to pause for navigational changes or where there's inhabited planets so ships would be coming and going. So, place to find salvage #1: star systems where there's a reason for shipping to stop or pass through.
Deep space stations used for research or waypoints or just because you need a reason in your story to let ships stop would also work.
Battlefields... but this is most likely also to be local to a star system or other "fixed point in space" (if that phrase can be used in regard to space). I personally suspect space battles will never take place in the void. Why would they? Unless the two sides agreed to meet at some lonely spot, there'd never be a reason to even be there. And you'd never know someone was there thanks to information being limited to the speed of light (unless you allow FTL).
In other words, salvage shouldn't be assumed to be just anywhere. Oh, it could be... ships go off course. Mysterious wormholes appear out of nowhere... But 99% of salvage will occur in predictable locations, and that means things like Radar suddenly have some value. Rather than waiting for years to find a needle in a massive haystack, you're waiting hours (and hoping the derelict isn't so close to the sun as to interfere with the signal...).
And you need to consider what it means to become a derelict
What causes a ship to become derelict? The Apollo 13 moon mission demonstrates the theoretical possibility of a ship being primarily intact even though everyone one it could be dead. So you have the case of a broken resource (like oxygen) leaving the ship intact. But piracy? A battle? A meteor impact? I'll be honest with you. I'd avoid reality when it comes to what makes a ship derelict, because IMO most causes would leave chunks-too-small-to-be-worth-salvaging spinning off into many directions that, with every passing second, are getting too far apart to effectively salvage.
Although that does bring up a point... until they start getting affected by another force (like a star's gravity), all you'd need to do is find two such chunks and trace their route and you'd find the source of the accident and, therefore, be able to predict where other chunks might have gone... Hm....
But, to be fair, that wasn't your question.