How to Find Hidden Cameras and Spy Gear
Doesn't mention the good old <i>Nonlinear Junction Detector</i>.
(NLJD) [1] or <i>The Broom</i> as they call it. It can find any unshielded  device containing semiconductors, no matter if the device is powered or not - which is pretty cool. It is possible to build one yourself which is even cooler. Although I have to say, while it is a WW II era device, DIY building is not a beginners project. The British documentary <i>"The Spying Game - Walls Have Ears"</i> has a good interview with its inventor that even includes a demo of the device by him in his wonderful old school lab[2].<p>[1] <a href="https://en.wikipedia.org/wiki/Nonlinear_junction_detector" rel="nofollow">https://en.wikipedia.org/wiki/Nonlinear_junction_detector</a><p>[2] <a href="https://www.youtube.com/watch?v=4EWYI9zfF9Y#t=22m30s" rel="nofollow">https://www.youtube.com/watch?v=4EWYI9zfF9Y#t=22m30s</a>