Adult rescue dog peeing sort of randomly

Rusty is almost 2 years old, male 40lbs. We think that he is a blue heeler/ Aussie mix And he is the perfect gentleman and very well-trained. We adopted him when he was about a year old. We found him at a local humane society, and when we first got him he had some inside accidents peeing and we (and our vet) thought that it was submissive peeing. Our vet explained that puppies sometimes will pee on themselves to show how submissive they are and that Rusty probably just hadn't grown out of it, so we figured that he would grow out of it. For a while it seemed like he did, but lately it's been coming back and more often. It's kind of random when he pees, there doesn't seem to be a real pattern to it. Sometimes it seems like he's nervous, sometimes it seems like he's excited and sometimes we just call him or walk in the door- but we also do all these things plenty of times without him peeing. He is a super big sweetheart! He's a lover big time and very attached. I think that he had a pretty rough start to life so for a while I just that as he grew more comfortable in her house the peeing would go away since we assumed it was submissive peeing but now I'm not so sure. He will even sometimes pee on his own bed. He definitely looks ashamed but we never punish him for it. Anyone have any suggestions for what the reason might be or possible causes? Does anyone have a dog that's still submissive pees at 2 years old? Any training suggestions?