Okay, I might be an outsider voice here, but I'm a serial-flirt and I've turned my social awkwardness from childhood into something I believe is now pretty charming.
So different tack: first, what was the event? Second, if you see them again and you decide they are cool and likeable and you'd still be chill getting to know them better, even if they AREN'T even at all a little bit poly, you might want to be a little more smooth about getting to the bottom of this info.
I mean, I don't know about you, but my in my experience with straight-vanilla folk, anything non-vanilla has the potential to be awkward at best, and social suicide at the other peak. Like, maybe they won't run for their lives. And maybe if they do, it would be making the inevitable happen sooner.
But also maybe they are super-cool fun chill peeps you could absolutely drink and play cards with and never even discuss relationships or polyamory or your sex life at all, and have a completely great friendship/relationship. And maybe sex is on the table. I'm a firm believer that there's never really a need to rush.
AAAAAANNNNNDDD take all of that with a grain of salt, because if I were in a similar position and was even at all a little bit tipsy, I'd probably straight put my foot in my mouth, and maybe regret it, or maybe not. Haha.
I was at an app launch party years ago, YEARS, before my marriage was open, but discussions had been part of our relationship for sure by that point. Anywho, I met a cute guy at the app launch party. It was an app for board game meetups, so the whole thing was centered around food and drinks and PLAYING GAMES TOGETHER. (Sidenote: it didn't take off at all, the app, I mean. Kind of a bummer, it was fun.) And this guy was SO FLIRTY with me. I made jokes about the frat he must have been in in college (and wasn't wrong). I made jokes about every college box I could put him in, in fact, because the vibe that night was so college-y and I was into it, and he was FLIRTING.
I made a joke about swinging. He was cool and it didn't land weird at all, but wasn't really picked up. Days later we connect on FB and it turns out his aunt is my neighbor, he's a BIG TIME church guy and is well known throughout our small town as a semi-super Christian. SUPER monogamous, obviously. Kind of an evangelist, if I'm being even more honest. And also runs a non-profit for networking college kids and helping them get jobs.
His entire personality was "flirty" for Jesus and his job. He wasn't, in fact, into me, even a little bit.
I was maybe a tiny bit horrified. The end.