Presumably when he saw that you were sending PMs, he also saw that your screen name is PK. Nothing is stopping him from making his own account here and reading all of your posts. (Even if he didn't see your screen name, I think he would start looking for you with a post called Spouse curious about me and this website . Might be a clue. Might be you leaving a trail of breadcrumbs...)
If anything you have ever posted in the public forum is still a secret from your husband, it's because he chose to leave it a secret. Or he's dumb as a fence post. (He was smart enough to hang onto you for 25 years, so I reject that notion.)
I encouraged my wife to make an account here and read what I was posting. So far she has made 2 accounts that I know of because she could not remember the first one. I have no idea if she logs on routinely to check up on me.