Hi, our cat is 18 years old, he still uses the cat litter tray in our house mainly for wee, on occasion poop (he prefers to go outside for this). Our litter tray is a fully enclosed litter tray but we noticed at the back of the litter tray marks on the wall. We moved it to another area along the same wall and the same has started to appear in this area also. As the tray is enclosed I can't see anything being sprayed on the wall. He doesn't sit atop the litter tray. Has anyone heard of this, does anyone have an ideas? Thank you.