The Thief

Detective Knightmare was staying at the Conundrum Hotel in Puzzletown. He was sitting on the bed in his room. He had just finished oiling his pistol when heard a knock at the door.

He was working on a case and had pictures lying all over the bed. He scooped up the pictures and placed them back in their folder. He slipped his pistol in to the drawer on the table beside the bed. He heard someone insert a key card into the lock. He hurried to the door and opened it.

"Oh, I'm sorry," said the stranger. "I must have gotten off the elevator at the wrong floor. I thought this was my room. They all look identical, you know."

The stranger laughed weakly, made a hand gesture towards the elevator, and headed in that direction.

Knightmare closed his door and immediately dialed the front desk of the hotel. He informed that a man was stalking the halls and up to no good. He described the man and suggested that hotel security should look for him.


Why did Knightmare suspect the stranger was up to no good?