Dr Watson was staying in a boarding house. Whilst resting in his room, he heard a knock at the door and opened it to find a stranger standing outside who said, "Oh, this isn't my room! Please pardon the intrusion" and promptly left. Watson went down to the owner to tell him that there was a burglar on the prowl.
What made him think that?
~~~
Answer to previous question: The neighbour was a photographer, developing a photo of his wife.

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Riddles | Sherlock Holmes
Mystery / ThrillerThe Case of the Curious Conundrums | A tricky test of lateral thinking Could your lateral thinking skills rival those of the master of elementary, Sherlock Holmes himself? Put your brain through its pages with this curated collection of riddles and...