Potty training tips for older dogs?

I picked up a 3 yr old Terrier from a rescue shelter about a month ago. Hadn't had and problems until today. My dog seemed to be potty trained and I found no evidence of misbehaving while he was left alone. He sometimes whines by the door when he needs to go out. So today I took him out as usual for a walk and he pooped and pee'd..... So then i had to go out for about an hour and when I returned I found that he pooped twice in the house. Now I am starting to wonder if he may have pee'd in the house on any other occasions. How do you really know for sure? And what should my reaction have been when I returned home? Should I have disciplined him some how? And now what do I do? Any tips ?