This is the central fallacy of the writer: he or she must absolutely believe something that is not true in order for it to become true.
Madness string 10931
Sure, you 'could have' shown him proof that the car was yours. You also could have put on a Spider-Man costume and run around the parking lot flapping your arms like a chicken and singing Twinkle Twinkle Little Star. The point is that it wasn't remotely within his rights to demand either of you, since you were a complete stranger to him, the car wasn't his, and he literally didn't have a single piece of evidence to suggest that anything inappropriate - much less anything illegal - was going on.