I kind of believe this as i got into a very similar situation at work the other week. I rocked up at 4:45pm to a department that normally closes (hours clearly posted on the door) at 5pm. The main door to the corridoor was open, but all others were locked.
"Hello? Anyone here?" I called. No answer.
I was busting for a wee and the loo was right There, so I dashed in quickly, reasoning that either they left the main door unlocked or else I had 15 minutes.
I came out all of 5 minutes later (4:51pm) and found myself locked in. The fire door proclaimed to be alarmed, but I needed to collect DS from childcare, so I thought "Fuck it" and prepared for the alarm to go off as I legged it down the fire escape. I got in my best runners stance and flung the door open and...
Nothing. Not a fucking thing. I legged it down the fire escape anyway, lest anyone see me and hid in the safety of my car. I then rang the main switchboard anonymously and got put through to security who barely believed me to ask them to go and secure the fire escape.
My first thought was not to post on MN because I escaped .
Moral of the story: Beware needing the loo in unfamiliar territory! 