jcom If an adversary knows your PIN and has physical access to your device while unattended, what technical approach could be used to reliably inform you after the fact that your device had been accessed?
Currently none. You're assuming that the device you return to is still the device you expect it to be physically, but with potentially tampered software/data. This assumption is unfounded. No matter what software you use on the device, it won't be able to save you from a fake device set up to mimic your real device, or from physical modifications like an independent microphone+antenna module inserted into the shell of your device, or a microphone being implanted in the room, etc. If you believe an adversary had physical access to your stuff, the correct approach in my opinion would be to throw the device to the trash and not be in the room anymore.
The GrapheneOS website says that they want attestation to also include the serial number. Since the attestation verifiably comes from an authentic Google Pixel Titan M2 chip, this could be used to know that it's the device you expect. But it won't be able to detect the antenna+microphone as I said above. And you won't be able to use it without first unlocking the device with your credentials. Actually, I don't see how it's possible for you to know that the device that sends attestation info (e.g. using the Auditor app) is the authentic device without you authenticating to it first, and then the device you're authenticating to could be a fake device proxing your authentication to your real device somewhere, and you have no idea to know it's proxying it before authenticating yourself and revealing your credentials. If you're relying on attested info that doesn't require you authenticating towards your device to see it, you're relying on info that an attacker could also access from your real device and proxy on a fake device mimicking what you expect your device to show you.