UTC 0830 Smart devices mimic what they hear around them so they can blend in. For example, for wifi using the same MAC address and IP info as something transmitting frequently. You'd have to check the sequence numbers or use multi-directional gear to detect it reliably.

Conceivably it would use a different waveform with lower energy when transmitting clandestinely, but that would be insanely expensive to achieve, probably need to add a separate chip, and it wouldn't work for people who change phones regularly.

I'm reminded that a while back the Shin Bet modified a phone to have a chunk of plastic explosive, used to kill a Hamas bomb maker. It was detonated remotely with a non-phone protocol (as the phone channel was in use by the target).

Agree, if the signal is strong enough to reach a cell tower there is no way you could hide it from someone who was looking for it.

I mean in the "affect airplanes" sense. Obviously it is distinguishable from noise because it has to be received...