People do try and stow away at Heathrow.
I remember working when a BA 747 was lined up on the threshold of 27L about to go, and another a/c waiting at the hold came on frequency warning of a man who had scaled the perimeter fence by Hatton Cross and was sprinting towards the 747. The man disappeared up into the nosewheel bay.
Eventually he was apprehended by a combined Police and Airport Ops rugby scrum!