
Visible crew/equipment: At the Quidditch match, after Hermione sets Snape's cloak on fire and someone shouts, "Fire! You're on fire!" there is a close-up of Harry with his hands gripping the Nimbus 2000. In this shot, the wire rods which are attached/taped to the broomstick, near Harry's fingers, that lead under both of his red sleeves, are entirely visible. (01:21:35)
Answer: Petunia or Vernon would have ran to a zoo employee and told them what happened. The employee would then open the door to the snake exhibit letting Dudley out.