I may be wrong, but it strikes me as strange that the refs would give the balls to the ballboys back in refs locker rooms (or whatever you want to call it). I figure if the league was even slightly concerned about security of the balls they would bring them up to the field themselves, then turn them over to the ballboys (in front of cameras and plenty of people).
If there was a sting, and this was indeed a deviation from SOP (by the refs), it could be part of the set up. Regardless of the ballboy using the can.