Comment by NoGravitas

You can set GrapheneOS to reboot into before-first-unlock state if it hasn't been unlocked within a certain period of time, and BFU is more exploit resistant, and can't be biometrically unlocked. I have it set to 18 hours, but you can set it anywhere from 10 minutes to 72 hours.

So yeah, as a US citizen re-entering the country, you would probably want to set it to 10 minutes, or just shut down. Then hand it over, refuse to unlock, consider the phone lost.