“I wonder what that fellow is looking for?” I asked, pointing to a stalwart, plainly-dressed in- dividual who was walking slowly down the other side of the street, looking anxiously at the numbers.
He had a large blue envelope in his hand, and was evidently the bearer of a message.
“You mean the retired sergeant of Marines,” said Sherlock Holmes.
“Brag and bounce!” thought I to myself. “He knows that I cannot verify his guess.”
The thought had hardly passed through my mind when the man whom we were watching caught sight of the number on our door, and ran rapidly across the roadway. We heard a loud knock, a deep voice below, and heavy steps ascending the stair.
“For Mr. Sherlock Holmes,” he said, stepping into the room and handing my friend the letter.
Here was an opportunity of taking the conceit out of him. He little thought of this when he made that random shot. “May I ask, my lad,” I said, in the blandest voice, “what your trade may be?”
“Commissionaire, sir,” he said, gruffly. “Uni- form away for repairs.”
“And you were?” I asked, with a slightly mali- cious glance at my companion.
“A sergeant, sir, Royal Marine Light Infantry, sir. No answer? Right, sir.”
He clicked his heels together, raised his hand in a salute, and was gone.

A Study In Scarlet, Chapter 2