[WrongOnInternet]

A bit more from the book (which is a great read, and available in it's entirety on archive.org): "To enable its agents to communicate over greater distances, the government had installed “repeaters” at high elevations to relay the signals. The agents’ radios transmitted on one frequency and received on another; the repeaters had an input frequency to receive the agents’ transmissions, and an output frequency that the agents listened on. When I wanted to know if an agent was nearby, I simply monitored the signal strength on the repeater’s input frequency. That setup enabled me to play a little game. Whenever I heard any hiss of communication..."

[lazide]

Properly encrypted data is indistinguishable from random noise - aka ‘hiss’. If really good encrytion, it will be white noise (generally). Albeit will have more power.

If there is a clear pattern to it, then that’s either unencrypted framing, or bad encryption. (Think 90’s cable TV ‘scrambling’).

[dogcow]

Not really true on modern digital radio systems. They are AES-256, but the voice frames are encrypted right after the vocoder does its thing, then the voice data is dropped into the stream just as if it were clear voice. It's all wrapped in the same same digital protocol (like P25 or numerous others), so the signal is very distinct in that encrypted and clear communications both sound the same to someone listening to the raw audio.