[justanotheranon]

Non-Linear Junction Detector.

https://hackaday.com/2017/09/20/spy-tech-nonlinear-junction-...

the math/physics behind how this works is fascinating, however, NLJDs are expensive, slow and require specialized electronics skills that the average person doesnt have. NLJD is what Military/Intelligence use to sweep a room for bugs.

but at end of the day, it's an arms race where the only way to win is not to play. if it became easy to detect circuits hidden in the walls or in household objects, they can just escalate to tactics like when the KGB used special screws in the beams of the US embassy in Moscow to record ambient audio of the keys pressed on teletypes to recover the messages. then there's the viral video from a few year back demonstrating recovering audio inside a room with a camera aimed through a glass window pane at a foil bag of chips. the same techniques that recover audio also work for video. it's just a signal to reconstruct.