Oh my... I think if the first thought is "one of the parents" they must have been very enthusiastic drinkers indeed!
it was the dog that had been caught short and had the good manners to use the bathroom.
A bit like the scene from "Meet the Parents" with the cat using the toilet ;D
My father already had form. More than once he fell asleep on the toilet, toppled off, and ended up wedged against the door so nobody could get into the bathroom until he finally resurfaced the next morning