I once worked with a guy who used to clean the planes before a departure. He said stuff was found all the time, mostly new purchases from duty free that people had forgotten about.
He said there was really very little chance to take anything as they were security screened on entering and leaving the airport.
He did say that anything that was found was tagged against the finder's name and was up for grabs if unclaimed 6 months later. Not sure whether the process is the same around the UK or elsewhere in the world.
I would suspect a passenger over a cleaner!