I wasn't there at the time this happened, but the story goes something like this...
Some years ago at EGLL the Tower rec'd a call from the met observer on the North side of the airfield to say they had spotted a "Chipmunk"on the northside. A subsequent call to Heathrow Airport Ltd resulted in the animal unit being dispatched to deal with said rodent.
20 minutes later or so the phone rang in the tower to say there was in fact a "chipmunk" on the northside, it did however have wings and an engine!! Cue some embarrassment from ATC as to why no one had spotted a single engine plane that had been taken for a joyride from a small local airfield and had landed unseen in the middle of the night!!!