The Vault by Murray Leinster

The window slid up easily–too easily–and Mike waited a long time, listening, before he made a move. The whole huge pile of the factory was still. There were no lights anywhere, except that dim one by the gate through the stockade. Lying quite still in the darkness, Mike waited. There was no sound, no ringing of alarm bells, no bustle of activity anywhere. The manufacturing plant of the Whitney Jewelry & Watch Company remained as it had been before, a vast, still pile of brick, with empty-eyed windows staring blankly at the night.

And yet. That window had opened very easily. Mike meditated, his little eyes gleaming in the darkness. Then he saw a tiny flicker of light in the distance. The window he had opened was at the end of a long corridor, and he saw the watchman walking unhurriedly away from him. The watchman's legs threw monstrous shadows from the lantern he carried, Mike could not see his face, but he could see the uniform and note the absolute leisure and confidence with which the man was moving. He paused, as Mike watched, and inserted his key in a watchman's clock. He turned it, registering his presence and vigilance on a strip of paper within the mechanism. Then, casually, he went on his way. In a few moments he turned a corner and was lost to sight.