I can top these, but it doesn't involve a FBO. It does involve the government though.
I was once sent on a service call to a not-to-be-named government institution (It starts with an "S" and ends with an "Ocial Security") to investigate intermittent failure of a signal booster. Upon arrival I was directed to a large storage room at the back of the building where the booster was located. The ceiling was about 30 feet high, and there was a maintenance catwalk that ran the perimeter of the room. The booster was wire-tied to a ceiling support about a third the way across the room. So I climbed out on the support (there was only about a 9-foot drop or so to the top of some shelving units if I fell, and I am a real small person. I wasn't too concerned) and proceeded to troubleshoot. In the course of doing this I got my cellphone out and called across the building to the government's network guy to have him test the unit while I worked on it. After about 15 minutes of this, I was waiting for him to do something or another when two employees entered the room, looked around (but not up!) to make sure nobody was around, then commenced to lock the door, lay a blanket down on the (cement) floor, and have sex. I was planning to ignore them but she started moaning and making noise, which could be heard by the idiot on the other end of the phone, and I was feeling like being an asshole besides, so I let it continue a second or two and then yelled "Hey, take that sh-t somewhere else! Some of us have a job to do!"
She screamed like she'd been shot or something, and they just about pulled a shelving unit down on themselves panicing. She started crying and ran off somewhere to hide, and the guy tried to offer me $200 to not tell anyone about it, but by this time people had heard the commotion and discovered the door locked, and were getting a someone to unlock it. The jig was up. The girl was a secretary and got fired, but the guy was a manager and was simply transferred elsewhere. I returned to the same place later for a different service call, and the people there remembered the incident and said they were happy to be rid of the guy because he was an asshole besides.