This is all so helpful. Thank you all for taking the time to answer.
I called the vet regarding a possible UTI - they asked me to get a sample - have tried and he looks at me like I'm crazy (guess I would too if I was in his shoes). I have wondered about separation anxiety - this guy has been in 3 different homes in 4 weeks (the original owner, foster and now his hopefully forever home). I especially suspected this after he had only been left for an hour. No accidents whatsoever if we are home.
Good news, however, I left to let him out at 12:30 and NO accidents so he was alone for a good 4+ hours!
Appreciate all the encouragement!