Suspicious Mystery Riddle

Suspicious Mystery Riddle Solution - 2 January

A girl was sitting in her hotel room when she heard a knock on the door. She opened the door and found that a man was standing outside.
The man said, "Oh! I am really sorry, I thought this was my room."
He then walked through the corridor to the elevator. The girl did not know the man. She closed her door and called the security asking them to apprehend the man.
What made her suspicious of that man? He might have been genuinely mistaken.

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The suspicion was natural. If the man thought the room was his, he would have tried the keys and not knocked on the door.