The Maltese Falcon by Dashiell Hammett:
“I got it all right,” Spade told Brigid O’Shaughnessy, “but Mrs. Flitcraft never did. She thought it was silly. Maybe it was. Post Street was empty when Spade issued into it. He walked east a block, cross the street, walked west two blocks on the other side, recrossed it, and returned to his building without having seen anyone except two mechanics working on a car in a garage.
When he opened his apartment-door Brigid O’Shaughnessy was standing at the bend in the passageway, holding Cairo’s pistol straight down at her side.