If it's a signal it will be noticeable with a tool that scans the relevant bands. Trivially so.
In what sense could it ever be "practicable unnoticeable", much less "for any equipment"?
In what sense could it ever be "practicable unnoticeable", much less "for any equipment"?
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).