edward snowden himself demonstrated a phone case about 13 years ago that showed that even in airplane mode and even when turned off most phones emit radio signals
welcome to 13 years ago, it's way way way worse now
Discussion
this is true of google android and iOs, however I understand it is NOT the case for graphene OS
but you have verified gaussmeter evidence that it is true?
I don't think you'd use a gaussmeter but rather a mobile base station so you can measure the incoming pings
This was the approach used by SRLabs when testing their Blue Merle software for IMEI switching
https://github.com/srlabs/blue-merle?tab=readme-ov-file#imei-randomization
doesn't necessarily have to be mobile radio protocol, just something that someone can pick up, doesn't have to be strong, about 20 metres is probably enough, and that won't detect it
The only way is to remove the battery, which was easy on cell phones 20 years ago. Now the quickest and easiest way would be to destroy the phone