The Dain Curse by Dashiell Hammett

I put in a couple of hours canvassing the neighborhood, trying to place the man Mrs. and Miss Leggett had seen. I didn't have any luck with that one, but I picked up news of another. A Mrs. Priestly -- a pale semi-invalid who lived three doors below the Leggetts -- gave me the first line on him.

Mrs. Priestly often sat at a front window at night when she couldn't sleep. On two of these nights she had seen the man. She said he was a tall man, and young, she thought, and he walked with his head thrust forward. The street was too poorly lighted for her to describe his coloring and clothes.