Is this a real issue for you? I would think that you would not normally leave your phone somewhere that it could be covertly accessed by an attacker. My device is almost always by my side.

If you're in a position that your chain of custody is broken, just wipe, revert the device to stock signed binaries, confirm by locking the boot loader again, then unlock and re-flash your custom binaries.

Or am I missing something here?