The Thing [2] is also worth reading. Soviet Union was well-aware that RF/LO leakage was being constantly monitored by the NSA, so they made a passive eavesdropping device in 1945, that didn't use a power supply. Instead, when eavesdropping was needed, an agent near the field would transmit an unmodulated carrier wave at 330 MHz, which was received by The Thing with an antenna and tuned circuit, thus activates the device. The recording is then rebroadcasted at a higher harmonic frequency. When the device is inactive, it's almost impossible to detect.
[2] https://en.wikipedia.org/wiki/The_Thing_(listening_device)