David was sitting in his hotel room reading a book, when he heard a knock at the door, and the sound of someone trying the door handle. He opened the door, where he saw a man whom he had never seen before. The man said, "I'm terribly sorry... I thought this was my room. Wrong floor, I guess. He then walked
off down the hall toward the elevator. David went back into his room, thought about what had just happened, and then phoned the front desk, suggesting they
call the police, since the man at the door had obviously been a thief.
Why was David so sure the man had been a thief?