[jeffdubin]

When troubleshooting wiring issues, a common tool used to find a specific wire pair in a bundle is the tone and probe set. The tone tool injects tones ('deedle deedle deedle') at one end of the specific wire pair you're trying to trace, then you wave the probe (also called an inductive amplifier) around the far end of the bundle of wires and listen for the tones. As you get closer to the wire you're tracing, the tones become louder.

This inductive amplifier also converts all sorts of other EM fields generated by most electronic equipment into audible noises. CPUs, logic circuits, etc. will be heard as 'tick tick buzzzzz tick buzzz' or similar clicks and squeals. One big caveat is that the range of most EM fields picked-up is only maybe a foot or so. But if you were to run a probe up and down a wall or around objects you suspect might contain a recording device, you'd likely hear it.

This isn't the only tool I'd use, but it's a quick and easy way to get a basic idea of what might be hiding.