If at home you'll probably have to get a temporary ID (paper) that the OOD can issue from the main gate of whatever camp. HMS Drake main gate does them for Guzz dockyard. I've had one from Nelson in Pompey before.
If its just on board then just get an oppo to vouch as others have said.
Report to OOD when you get back. It's different for lost than misplaced. Take your MAA like a man (or woman): the leaves won't sweep themselves up you know.
Ahhh.... urination in an improper place. If I had a pound for every matelot I've been to table with for that. Used to use up most of every Wednesday morning at COLLINGRAD for that. So well done on the Army's gift of MAA.
Mind you, every time I go through the doors of COLLINGRAD I feel like peeing all over the place.....