Bug Detector

These circuits are much more complex actually, a good one that is. You need a properly tuned LC input filter, then a high gain amplifier, because an rf signal is rarely above the uV range, extremely rarely reaching mV, and that's if you're very close to the transmitter. Anyways that's the hard part, the rest is just a peak detector and a comparator. The idea behind it all is to make it either tunable over a wide bandwidth or make it a broadband detector. Both designs exist, but the tunable one is usually better, because you can track the specific frequency, whereas the other design may not even pick the exact frequency if there's another strong rf signal nearby.

I think spectrum analyzer is the fast way governments use for showing transmitters in use.

Radio foxhunts have ways to detune antennas when close to transmitter, I have been looking into those rigs.