I used to work as a policeman on a military base. I loved working the night shift, but weird things sometimes happened.
One night, I was on patrol with my partner when we went to an elementary school to investigate a triggered alarm. We looked around, but everything was in order, so we left and continued on with our patrol. About 15-20 minutes later, the alarm at the school went off again. We went back to check and saw an open maintenance door leading to the boiler. There was nothing there, so we closed the door and blocked it.
Twenty minutes later, the alarm went off again. We returned to the school, but there was no one there. Half an hour later, the alarm rang out again. We went back and once again found nothing. This went on until three in the morning when we finally had to call the custodian of the building for help. We searched every corner of the school until we eventually found an open maintenance door. It led to a tiny room, about the size of a closet, and the light was on inside.