Why does my 11 year old mini schnauzer leave poop balls while sleeping

My 11 year old male ms has begun dropping poop balls in his sleep. It's not intentional and I don't think he knows he's doing it. This started after he had some small tumors removed 5 months ago. They look like little round balls and they are hard. Before the surgery I changed his food to taste of the wild and thought the balls may be an after effect of the surgery or the food. I switched to California naturals and the problem persisted. A few weeks ago I switched to science diet sensitive stomach. In the beginning it would sometimes be 3-4 small hard balls; with the science diet it's usually one ball but softer and a little larger. The vet said there's nothing wrong; only maybe a little incontenence but there was nothing he can do. Please share any suggestions on how to address this problem. I'm interested in anything that might help.