Even “off” many devices are still energized at some level;as said by another:if it is turned on or off by software detecting a button it can be turned on or off by software ,either hacked or with capabilities unknown to the end user.
Placingthe cellphone in a foil bag or metal canister with lid is one of the surest ways to be certain it isn’t still communicating your location.But it will acquire your new location onbringing it out.
Anonymity is truly rare.
Once again I’m glad I don’t have one of those “smartphones” .... I have a stupid “feature” phone. It makes phone calls and that’s it. No GPS, no text, no email, no web browsing. The only way to track me will be via the cellphone towers, which is still a pretty big area.