Just as I finished the last post, I thought I heard a hair ball being coughed up.
When I went look, no hair ball, but some funny cat-expelled liquid (pre-hairball?) was in the track of the Turbo-Ball in the library, and someone (perhaps the same cat), had a left bowel movement on the rug.
The latter is very troublesome, because both the Catzilla brothers and the Grey Mouser always use the litter box.
(The Mouser being perfectly litter box trained was one of his endearing qualities. The boys miss the target some times, but that's different issue from not using it.)
For someone to ignore the box may be a stress level problem, and that could be really hard to fix.