I’ve been in contract security for 20+ years. A fair amount of that time was working at posts, some of which were relitively old buildings for the area.

All of them had their ghost stories.

My favorite was the 1890-built building that had a ghost that would walk down the stairwells every night at about an hour before midnight.

(Reality: it was the standpipe running up the stairs that fed the fire sprinklers. As the building cooled, they knocked against the stairwell landings.)

Another building, the doors would close and lock on you. They’d not unlock. This was known to happen “since I’ve been here at least” (a guard that was showing me the account, who had been there for a decade.)

Turns out it was the magnetic door holders and electric strike locks. There was a short in the system that would cause intermittent faults causing the mag holders to release (because “Fire!”) and fail-to-locked latches to… fail to locked…

(Also, this is why you don’t turn off fault reporting in access control systems…)

Super common is when you see double images looking through multi-pane windows. Usually those are reflections of the person seeing them, and look like that reflection is coming towards you, only exists for a moment and is usually blurry.