How to Find Hidden Cameras and Spy Gear
Doesn't mention the good old Nonlinear Junction Detector.
(NLJD) [1] or The Broom 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 "The Spying Game - Walls Have Ears" has a good interview with its inventor that even includes a demo of the device by him in his wonderful old school lab[2].[1] https://en.wikipedia.org/wiki/Nonlinear_junction_detector[2] https://www.youtube.com/watch?v=4EWYI9zfF9Y#t=22m30s