I have a 2 yr old Beagle mix that came from the pound last year. I have had trouble house breaking him.?

Any change to his schedule and he goes in the house. My work schedule/house guest. I thought I was making real progress, then he started going for his nightly walk and going outside. He will come inside and within 20 minutes have another bowel movement in the house. No warning.. He was just to the vet but could there be a medical reason or just behavior. First time dog owner... Help