Why these exist
Every other detection method depends on the device doing something. A signal detector needs it to transmit. A lens detector needs line of sight to the optics. A device switched off inside a wall cavity, a piece of furniture or a power outlet defeats both.
A non-linear junction detector transmits a signal and listens for the harmonics that semiconductor junctions produce when energised. Almost any circuit board responds, whether powered or not. That is why these are standard equipment for professional sweeps and why they cost substantially more than consumer detectors.
Who uses them
Corporate security teams before board meetings, legal and financial firms handling sensitive matters, and professionals conducting sweeps for clients. If you are checking your own home or car occasionally, a combined signal detector and lens detector is the appropriate tool.
Common questions
Does it need the device to be on? No. That is the entire point, it finds electronics regardless of power state.
Will it react to ordinary electronics? Yes. Interpreting the response is what the training and the price are for.