My 15 year old Thomas has always been the most well-behaved cat you can imagine. Just a couple of months ago, he started urinating outside his box. I think he might have a little arthritis, so I got him a large, low edged box so he wouldn't have to step up or turn around much. Not much improvement. Then I started keeping a closer eye on him. I discovered that what he's doing is stepping into the box, then spraying outside it. (Tail straight up, twitching.) I've taken him to the vet. Urinalysis shows absolutely nothing wrong. There have been no changes in our home. His life is exactly the same as it's always been. And other than that, his behavior hasn't changed at all - still very social, appetite is fine, playful and loving as ever. I've put puppy house training pads around the box to aid in clean up, but does anyone have any insight into what might be going on with him?