That's a strange story, Smee. Like you, I'd definitely want to get in touch with the buyer and find out subtly what has happened. It might indeed be a case of the buyer having neglected to pick up the item after being carded, only to find out when he/she finally went to pick up the item it had already gone back to the sender... and thinking, "Oh no! It's my fault! I'd better at least give the seller positive feedback, and not mention how foolish I've been."
People do amazing things to avoid admitting their folly. Sometimes we simply have to admit we do stupid things... We're human...
I would certainly expect the buyer to pay for the postage again, though, if this turns out to have been the scenario...!