Thanx for responding. The litter box has a filter on it, and we clean it right after he poops. We know he does it twice a day, 6-7 am and then 10-11 pm, so we always clean it within few minutes. Also, we have a liner undrerneath a litter and we change it along with litter every week. There was only one time when we left the poop there for couple of hours, and I am wondering if that may have triggered this behavior. Maybe he thinks that if he poops there again it will smell but it's still safe to pee in there. But again, he is only 4 months old, so it's like 2yo baby, can he be that smart?