Wouldn’t turning the phone off accomplish the same thing?

All of the bags and boxes others have posted make the phone unusable, so turning it off seems just as well. Plus, if you leave the phone on, without airplane mode, in a faraday cage, it’s going to die rather quickly while it searches for a signal.

Believe it or not, no - modern phones are rarely genuinely “off.” iOS has some form of “find my” that still works, I’m not sure where android’s at with it, but “off” and “not transmitting” are genuinely not the same thing at this point.