I believe they have been monogamous the whole time they're together, at least as far as I can see. The woman I've known longest, who is bi, was in open relationships before she met her gf. GF is definitely women-only, and I have no idea if she has experience with polyamory.
I'm going to wait until I see them again (I'm sure I"ll catch them at a holiday party) then see if the vibe is still there. We were all pretty far gone at the party where I thought they came onto me, I want to make sure it wasn't purely chemical-induced lust. If I'm still getting signals, I'll say something.