Murder in the Gunroom by H. Beam Piper

When he rose, the next morning, Rand noticed something which had escaped his eye when he had gone to bed the night before. His .38-special, in its shoulder-holster, was lying on the dresser; he had not bothered putting it on when he had gone to see Rivers the morning before, and it had lain there all the previous day. He distinctly remembered having moved it, shortly after dinner, when he had gone to his room for some notes he had made on the collection.

However, between that time and the present it had managed to flop itself over; the holster was now lying back-up. Intrigued by such a remarkable accomplishment in an inanimate object, Rand crossed the room in the dress-of-nature in which he slept and looked more closely at it, receiving a second and considerably more severe surprise. The revolver in the holster was not his own.