Having been employed by BA at LHR for a considerable amount of time I can say that it was a term that was widely understood as going home before your shift ended for reasons other than business, personal, official leave/lieu day or sickness. I hadn't come across the term before then, when I was in the RN.
It's a fine carrot to dangle but can be counter productive when traps are expected as a right.
Time to trap.......