Oh you poor thing. What a bastard! My DH went through this, as well. His GF before me (or maybe the one before that, I get them confused), she did this. They lived together. She had a previous coke habit, but seemed to be over it. She was quite insecure. Anyway, he went on a rare lads night out, she couldn't handle it, and by the time he got home, she'd gone...totally unreachable for weeks on end....he didnt know if she was dead or alive....he eventually managed to contact her parents who confirmed she had slipped back in to her coke habit (which unfortunately went hand in hand with fucking random men) and he never saw her again....no explanation, nothing! She was extremely fucked up, and I suspect your OH could be too
It's easy to say don't take him back, but I'm sure some part of you could be tempted, but he'd have to do a lot of crawling and convincing that this was a one time blip....which perhaps would be easier to believe if he had crawled home, rather than him just saying this when you went to his workplace.
Fwiw, my first H did similar, although we didn't live together, but 6 months in to our relationship, he suddenly declared he might still love his ex and want to be with her. It was fucking New Years Eve, and he left me to go to her. I can still remember sitting there in is house, dumbstruck as to what had just happened, I even still had my stupid paper party hat on! He came back after 24 hours, begging forgiveness, which I gave (I was only 17), and we went for 20 years! Then he cheated and I left him, but that's not the point here.
My Sister's DH did the same! Went back to his parents with jitters, only to re-appear a day later. They are now married, ten years in and happy.
So, I'm not sure which way you should go here. Trust your gut, I guess. He'd have to do a lot of grovelling....it could be salvagable, just maybe...