I always interpret this to mean he was screwing around, accidentally pulled the trigger while drawing it out of the holster, etc.
The act of ‘cleaning’ usually starts with disassembly (after clearing it of course). So unless all these guys are ‘cleaning’ the outside of the gun, its just a term that means “I screwed up but don’t want to admit it”.
Another possibility is he dropped the magazine and left a round in the chamber. I was in the military, and I can attest that some people, whose experience with pistols is limited to the qualifying range where you fire until the slide locks back, do not fully comprehend the concept of a round in the chamber.
Had to be put up in a tent with a bunch of 1st Cav
chopper pilots at a firebase once.
This Capt kept playing with his 45 and sure enough
it went off and put a round about 2 feet from my cot.
I always wondered how that guy made Officer.