"Goodnight, Frankie boy."
"Yeah, g'night, Bill."
The night watchman closed the door as his colleague left and slid his key into the latch, locking it behind him. From there it was all downhill. He wandered back down the long corridor of the ground floor offices and gradually switched all the lights off as he passed through until he reached the far end where his little security office was located. He was confident now that he was the only person left in the building.
He locked that door behind him too, as was procedure, confident in the large screen displaying CCTV footage throughout the building. He was just settling down with a crossword puzzle when he heard an unmistakably loud thud from out in the corridor. Stricken and confused, he scoured the screen, seeking the camera feed and, upon finding nothing, decided to investigate.
He had a long walk before he reached the corridor light switch and so, brought his flashlight with him, swinging it back and forth, looking to find the object that had fallen or the suspected assaliant, which lurked at the back of his mind. He scoured the walls, found himself feeling more nervous than usual. There was nothing. Even once he had switched the lights on, checking the various rooms along the corridor, he couldn't find anything.
He decided to leave the lights on for a couple of hours, making a mental note to come back out and shut them off on his next patrol. The building was forever spooking him, it was an old rickety place. Formerly a judicial office then a private lawyers office for half a century before the insurance firm had bought it over. It was incredibly eerie at night but he could usually put it out of his mind.
Once he made his way back to the little office, he closed the door and locked it once again, giving it a firm push to make sure it was tightly shut. He decided to have one last glance through the CCTV feed, surprised as he noticed a faint greenish light easing it's way down the corridor. This was like nothing he had seen before. It was almost translucent, only a faint, rectangular shape glowing against the shadows cast on the walls by the overhead lights.
He watched it glide down the corridor, fading out of view and then back in and grew convinced that it was an error within the camera itself. But as it progressed, slowly picking up pace, he realised the glowing shape was also present on the feed from outside his office door. With a bemused expression, a sense of dread bubbling in his throat, he watched as it slowly approached the camera, disappearing out of view.
Then he heard the unmistakable sound of the office door slowly unlocking.
(Credited to Scotty from Terror Tortellini)