Thanks again for all replies. I think my now ex OH and I have just gone through the final act in this drama.
This morning I went to the hospital he had supposedly been staying at, went to the admissions desk and said I was trying to get to see my brother (gave OH's name) who had been admitted last Thursday for an endoscopy/colonoscopy, as I'd not heard from him since. The first thing the receptionist said was, "Colonoscopy? That's usually done as a day surgery, he'd be long gone by now." She didn't want to tell me anything because of patient confidentiality, but I got (genuinely) a bit tearful and she asked for his DOB and address.
He's not been seen at that hospital at all, for any reason, since 2006.
They don't even have his current address, only the previous one that he moved out of about ten months ago.
So that was that, I left. OH had been conspicuously avoiding me all day, I finally got a text from him this afternoon asking if he and his son were still OK to come over for dinner/sleepover as prearranged. I said no, I wasn't feeling well, "for various reasons." He asked if he'd upset me in anyway, I replied, "You tell me, have you done anything I might be upset about, if I were to find out?" He said no.
Anyway, to cut a long story short, I drove to his house on a whim, he was in, I said that I was going to ask him one question and that he should think very carefully before answering. "Where were you last Thursday to last Sunday?" He thought for about 0.2 of a second and replied, "At the hospital." I said OK then, and just asked him to give me all the stuff I'd left at his house. He did so, very quietly. Then as I was getting back in the car he said, "Is that it then?" So I said, "Yeah, unless you've got an explanation about where you were." He said, "I was at the hospital." Maybe I shouldn't have told him but I did, I said I'd been into the hospital myself and that they had no record of him since 2006. Then I said, "Wherever you really were, I hope you had a lovely time," and drove off.
This was a good couple of hours ago now. Haven't heard from him since. I'm incredulous, and devastated. More incredulous I think. I couldn't even lie about the time I went to bed last night without having heart palpitations. How can someone do this?
Thanks again for advice and support, you've been great, I don't have many people to talk to in RL.