He tried to put himself in Cayley’s place⁠—Cayley, when Antony had first discovered him, hammering at the door and crying, “Let me in!” Whatever had happened inside the office, whoever had killed Robert, Cayley knew all about it, and knew that Mark was not inside, and had not escaped by the window. But it was necessary to Cayley’s plans⁠—to Mark’s plans if they were acting in concert⁠—that he should be thought so to have escaped. At some time, then, while he was hammering (the key in his pocket) at the locked door, he must suddenly have remembered⁠—with what a shock!⁠—that a mistake had been made. A window had not been left open!