I rescued my 5 yr old mix over a year ago - for the first several months he had severe separation anxiety -he was like velcro and had an accident practically every time we left the house (we couldn't crate him because he'd injure himself in an effort to get out). He was doing great for several months - we keep him very social and he goes to obedience school once a week, but recently he's begun to get very clingy and is going to the bathroom all over the house again! I don't think he's sick, but I can't figure this out. We left for a weekend about a week before this started, but we've done that before with no issues... any suggestions/ideas would be appreciated!

Have you ruled out any possibility of health problems?

Has anything else changed in the house recently? Changes in schedule, new pet, visitors, etc. Something that might not seem major to you but could be to him.

When you went away this last time, did the person taking care of him have any problems with him? Was it the same person as previous times? Have you asked about how well he/she kept to the established schedule?

Great questions... I have not ruled out health problems, but he was at the vet for a rabies vaccination about 2 months ago and the vet said he looked good all around, and within the last month I switched him back to K9 Advanitx (he was on another treatment for about 4 months and got fleas). He didn't have any problems the first year he was on it, and no problems during the few months he took another brand.
The woman who watched him sees him once a week and has watched him before. She has two cats, which she said he gets along with fine. She takes him for walks daily, but she does let him sleep in bed with her - which I rarely do, but have on occasion.
One thing that has changed is our next door neighbor (who had a beagle and a basset who were both very vocal) moved out, right around the time this started. It's probably a long shot, but maybe the silence from next door is upsetting him? My dog never really wanted much to do with the neighbor dogs, but he always knew they were there.

The problem is OBIEDIENCE SCHOOL. He's probably scared of other people and other dogs because they mistreated him, or he just misses you. Try and crate him one day, and see how it goes. Put cushions in any places he might hurt himself and put paper towels on the round just in case he has to go.