I remember the tale from one DAMO. I believe a prisoner was being escorted to RAF Germany for his court martial. He was checked in and then requested permission to go to the toilet. The sensitive rozzer uncuffed him and let him enter the toilet.
Some minutes later curiosity got the better of him and he went in after the prisoner of whom, no sight. Our eagle eyed upholder of law and order then noticed a ceiling panel had been disturbed. Undaunted he stood on the pan and peered in - to the ceiling void.
The prisoner was still there and could find no way out but apparently it took quite some time to catch him which relieved the tedium of those waiting for their flights.