Well...do how sure are that you want to go there, and do you feel pretty sure your friendship can withstand it if it turns out to be a misunderstanding?
Me personally, I'd probably just not say anything unless I was approached, with people who weren't out as being poly or open. Especially as you don't seem to be sure if he is bi or only bi-curious (don't have much experience with bi men, but do know that lots of women who are bi don't want to date bi-curious women - too much potential for heartbreak if it turns out you get feelings and it turns out they're all "eww yuck I guess your genitals aren't my thing after all, nevermind".
However if I did want to go there, and it was starting to eat at me not knowing, I don't think I'd drop hints, all that vague signal watching and getting other people's interpretations of what is going on gives me a headache and often doesn't seem to get anywhere when the reality of a situation finally comes out. I'd probably just come out and ask "Are you two thinking of dating outside your relationship? which gives him/them the chance to explain if there IS anything they are thinking of doing (dating, sex, swinging), then I'd take it from there.
Happiness will never come to those who fail to appreciate what they already have.