House Trained Dog Peeing Inside When Alone?

Our two year old dog that hasn't had an accident in the house since he was 6 months old has suddenly decided to start peeing in the house (on our couches) when we are gone. He has a doggy door and access to the backyard at all times. We've tried cleaning it with the pet odor removing cleaners, etc., but he keeps peeing. We are hesitant to punish him too much as we can't catch him in the act since we aren't home when he does it and he never does it while we're home. Any help on how to correct this would be great. We work away from home and we live in a freezing area, so crating him all day won't work and putting him outside all day won't work.