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.