I've noticed on IB on the modest amount of times I've travelled with them that they close the business lockers prior to boarding. This tends to prevent most Y chancers as many won't open what I guess is conceived to be a full locker. I'd like to see this method at least trialled on BA.
Someone had a gate checked bag with destination tag on a recent flight I've taken that they decided to bring on-board the aircraft, shoved up in the locker above my row and headed back to Y. Would this not have caused a mismatch in the number of checked bags on the manifest? Should it have been challenged by a ground/cabin crew member before departure? It got me wondering what the implications might have been if said passenger then put in a missed bag claim upon arrival and claiming all sorts of lovely things.... the bag tag could easily have been removed and there would have been no bag for BA to trace as the passenger already had it.