Anonymous thank you card through post!

Mr Spotty has just received a letter in the post tracked to him. The sender is from a student flat in Leicester. Inside was a badly generated folding card with the words “Thank you” on the front and nothing else, no name or signature. The tracked label on the envelope also looks rather home made. We don’t know anyone in Leicester and are not expecting any parcels through the post. We are scratching our heads over this and wondering what sort of scam it could be.