Stranger in the Hotel

From "Perplexing Lateral Thinking Puzzles" by Paul Sloane & Des MacHale.

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A woman was sitting in her hotel room when there was a knock at the door. She opened the door to see to see a man whom she had never seen before.

He said, "Oh I'm sorry. I've made a mistake. I thought this was my room."

He then went off down the corridor to the elevator.

The woman went back into her room and phoned the front desk to ask them to apprehend the man, who she was sure was a thief. What made her so sure?

Hints for Stranger in the Hotel!

Solution to Stranger in the Hotel!