another vote for prank from me
.... x 3
As Mat suggested, I reckon a mates' prank too. A very funny one too and rather less damaging than the one I witnessed at a local college where a bunch of guys were doing a HND(?) automotive course where a workshop was available:
One of the lads had a Ford Popular which he had bought very cheap and was in need of some attention - This was back in the early 1970's - And it was his birthday so he and his mates went out to a local pub for lunch. But not all of them went, making excuses of having something which needed doing and they'd follow later. It was a hot summer and the Ford Pop owner had whistfully commented earlier that he wished he had a convertible. So, you can guess what happened! When they all came back from lunch....... He had a Ford Popular convertible! I remember that he wasn't best pleased but the whole college thought it was hilarious.
Oh, this was up in Greeners neck of the woods: Bromsgrove, near Birmingham.