Back in the mid 1970’s I worked at a drug rehab for drug dependent teens. I normally worked the 3-11 shift but had decided to do a double since we had a young lady in the observation room. The facility had been an old WW I airforce base and a state mental institution before being changed to the live-in rehab. The observation room was on the second floor which had nothing except a very large dorm room and the small observation room. The nursing desk was just outside the door of the observation room. At the end of the large room was a metal door leading down to another metal door via metal stairs. All doors were locked and checked before my shift began. Exposed metal pipes were around the large dorm room which were for heating and cooling. At about 2 AM I am sitting at the desk and I hear the bottom door open and footsteps coming up the stairs. I am already on the phone to security for help. I see the door open at the end of the room and then close. In about 30 seconds a pounding started on the pipes. It was if someone was going around the room hitting the pipes with a stick. The pounding stopped and I saw the door open again and close. Needless to say I was petrified. Security was there within 3 minutes of my call and all the doors were still locked. That is one night I will never forget.