A local device like an iPhone has to know what AirTags are yours, since they have to guide to back to the beacon. If your iPhone notices beacons that appear at multiple locations, that aren't yours, it can detect that.
Apple knows who you are. Apple knows which airtag you're near. You don't know which airtag you're near. It seems pretty clear that apple can use this information to tell if an airtag was placed in your car unbeknownst to you. Not sure what the mystery is.
