Its the freak washing machine part . . . or just that the whole thing is utterly surreal.
But as I said, it IS genuinely horrible, and no, I can't imagine how anyone misses a monkey. The only thing I can think of is if the system is semi automated, like some dish washing facilities, that the cage was shoved into a loud, steam filled room and people just toss it on the conveyer belt, that you might be able to miss the monkey. Its the only way I can imagine it happening by accident. Honestly, I wonder if it was an accident.