This will be long so I will try to keep to minimal details, but OP asked if anyone has experienced similar.
My DH did similar many years ago. Packed a bag, walked out and vanished. I didn't find out for months where he was living, although he did not completely cut contact. He would occasionally turn up for an hour or two to see our slightly older kids and would answer text messages intermittently. I did everything in that time, looked after the house, garden, kids including one having surgery and GCSEs, got promoted etc. It was horrendous.
The whole time he denied that there was someone else. I didn't believe him, and yes, it turned out that there was. He swore blind it didn't start until after he left, but I am pretty confident he only meant the physical side of the relationship. All the evidence I found all strongly supported an emotional one at least. He looked like shit for the entire duration. Trying to dress and style himself like a younger man but stopped exercising, having always been fit. Etc. You get the picture. His mum knew he was with an OW and met her. She cut me off, didn't bother with our kids having always been very close to me and them previously. He kept OW away from all his and our friends, so they had no clue, and hung out with hers instead.
Once he knew that I knew I had been lied to, he actually moved away from her and lived on his own, while he processed having blown his life up for a fling. Right from the day he found out I knew, he immediately admitted it wasn't going to last because she wanted kids, but they hadn't bothered having that conversation upfront!
He turned up one day about 6 months later and said he was ready to come home. I said no. Over the next few months, we did a lot of talking and he went for counselling. I had already been going. Anyway, eventually I did agree to let him move home and although we are a long way down the line, there are still things that come up, wounds that will never quite heal.
Would I advise anyone else to go through this kind of reconciliation? Probably not. If I had been younger, I probably wouldn't have done it myself. As it was, my kids were old enough to have conversations with them and get their views as well as deciding what I felt. I also know I can walk away and be fine if I choose. To this day, I still don't really understand my decision to allow him back in, but it was right for me and I don't regret it, but do expect to get shit thrown at me here for it.
At the time, I found that between friends and neighbours, I knew of at least half a dozen couples where the husband had done something broadly similar and they had got back together several months or years later. This may have influenced my decision.
Conclusion? It's an OW, regardless of whether you ever find or get shown the evidence. It doesn't matter either way because beyond knowing it happened, the details of another relationship are unimportant. It is what you choose to do with that knowledge that matters.