Originally Posted by cassblonde
I've never heard of it happening on it's own either - so cool. I've been doing a lot of reading on toilet training and he may be doing it because water covers the smell of his pee better than litter does. That is one of the reasons people succeed with toilet training - the cats learn "Oh if I pee in the water it covers my scent better than the litter does!" so they start targeting the water.
Just an idea but if you are ever concerned about his urine output you could always place a bowl in the toilet he uses and collect his output for the day in it and recheck every so often to make sure it isn't increasing(which is a sign of illness). You would probably have to put water in the bowl and if it had measurements on it you could tell at a glance how many cups he peed in a day and his scent would still be covered so the smell wouldn't make him use another toilet or have an accident.
The OP could buy an urine hat for the toilet at a drug store , and that would tell them how much the cat peeing. I sure wish I knew how to train Marty how to use the toilet.