This happened a few years ago. I live in an area with a lot of people in open relationships… including one of my former coworkers and her bf.
She and I have always been pretty tight. We both worked the same closing shifts at the restaurant (neither of us work there any more.. both moved on to bigger and better things) Lots of late night drinks, “what does it all mean” kind of conversations–not just your typical jokey coworker relationship. we never hooked up, but we would definitely flirt a lot. She had a boyfriend and I knew that they had been together for a long while, but I didn’t know they had an open relationship. (even if I had known I probably would have held off on coming on to her because in my experience those relationships are very fraught…)
Anyway, she got a new job, and I was bummed because I lost my drinking buddy, but it happens all the time. I kind of figured I wouldn’t see her again, save one or two times that she came in to get some food or something. But, maybe a month later, she texted me and asked to get drinks.