You don't believe Apple would find out about them? They could know before the lease is signed.
Where would they put the code to do this? How would it function if Cellebrite put devices in a Faraday cage before transport?
Indeed you can’t rely on GPS. But…the absence of GPS for more than a few hours is pretty weird. An automatic reboot and maybe nuke could be appropriate for some users. I guess then the faraday cage also needs a fake gps signal to defeat this.
Apple also has their tracking network. Airtags + every Apple device
Probably not useful if you’re in a faraday cage.
If you have the power budget than you can compare accelerometer data to check validity of the GPS, dead reconing-style.