Current-day or near-future sensory technology is pretty much cameras and radar. These are basically the same thing, except applied to different parts of the electromagnetic spectrum.
This is good, because I can be lazy and only need to talk about one bit of science.
How realistic are these plot devices?
Fairly realistic - but for really boring and mundane reasons. Without any special forms of interference, we’d have a lot of trouble looking at the surface of planets in all but the most ideal situations. I imagine having little interference would raise more interest than having lots of it.
Gases, vapours, clouds, etc, all absorb parts of the electromagnetic spectrum and re-emit it as heat. Which parts of the spectrum are absorbed are determined by the chemical composition of the material (and in some cases, the size of the vapours too).
We choose to communicate and scan in parts of the spectrum where we're unlikely to find a lot of interference. Unless you're going somewhere specific, and are fore-warned of what atmosphere you'll be looking at, it's best to take the scout's motto to heart.
Be prepared for anything.
If I was a spaceship designer, I'd make sure I could communicate over as many parts of the spectrum as possible.
Scanning the composition of the atmosphere.
A common question asked by any spaceship crew would be can we breathe the atmosphere? A simple (but incomplete) test would be to measure the absorption spectra, and look for tell-tale indicators that you already have in a database.
Scanning for solid objects.
The best analogy is sonar (yes, submarines). You have passive, where you listen for noises/reflections/signals/etc. And you have active, were you make noise and listen for reflections of your noise.
On top of that, the frequency of the radiation you're using will affect the amount of detail you get. Loosely, the higher the frequency, the more detail. Lower frequency radio waves have longer wavelengths. Long wavelengths have this tendency to pass around smaller objects without interacting with them. This is true for all waves, including electromagnetic ones.
Picture a tall wooden post poking up out of the water. Imagine dropping a coin into the water next to it. The coin creates little ripples that bounce off the side of the post and reflect back out in the other direction. Now imagine a slow ocean wave rolling past – it doesn’t appear to be affected by the post in the slightest.
A similar thing happens with radio waves. If you want high-quality information about the surface of the planet, you’ll need a higher-frequency radar. But you might get unlucky and have atmosphere that absorbs exactly those radio waves.
The logic behind communication is very similar to scanning. You want to be able to receive information without interference and want to be able to transmit information without it too. You’d naturally choose similar parts of the spectrum to those of scanning.
This presents a bit of a conundrum with scanning. Communication might look like interference to scanners, and active scanners might produce interference to people communicating. Are your scanners capable of filtering-out communications?