Here’s an app note about a circuit for detecting and locating radio frequency transmitters. The circuit is based around the MAX2015 RF detector which outputs a voltage proportional to the strength of a received RF signal in the 100 MHz to 3 GHz range – [via]
This design idea showcases a circuit that detects RF “bugs,” such as hidden wireless cameras, eavesdropping microphones, and other spying devices that emit RF frequencies in the 100MHz to 3000MHz range. A modification to this circuit not only detects RF bugs, but also locates their hidden positions.
App note: Detecting and locating RF bugs - [Link]