When Peter is driving his car and picks up Buddy, he says he just found the kidnapper's car and is now bringing it to the police station. Buddy enters the car, and when he sees in the glove compartment the same chocolate bars he was given by the kidnapper, he then knows that Peter is the kidnapper. But if Peter was telling the truth and really just found the kidnapper's car, it would be no surprise for the chocolate bars to be there (as well as for other things belonging to the kidnapper).