Read "The Spy in the Elevator" by Donald E. Westlake online for free on Textopian. Full text with search, annotations, highlights, and AI-powered reading aids.
He couldn't have been anyone else but the spy. The gun, in the first place. The fact that he looked harried and upset and terribly nervous, in the second place. And, of course, the fact that he came from the elevator shaft.
Looking back, I think he must have been just as startled as I when we came face to face like that. We formed a brief tableau, both of us open-mouthed and wide-eyed.