it needs to draw power, so the ultimate way is to look for anything generating radio waves. there a few inductive methods to "listen" to electrical pulses near

it needs to draw power, so the ultimate way is to look for anything generating radio waves. there a few inductive methods to "listen" to electrical pulses near by, but you have to sweep every part of the room, take apart every source of EM. e.g.: light fixtures drawing power while turned off. same with wall sockets.