I don't know how to try and date people in my social circles if my non-monogamy is a semi-open secret

I've been dating the same person for 2 years and we've been poly the entire time. Our community and peers think we're slutty because we flirt with others. We're not necessarily out about not being mono but how do you even bring that up to people who aren't open minded anyways? I've been trying to date this other person recently but I can't tell how to bring up polyamory. They know I'm in a committed relationship and that my partner knows I'm openly flirting. I'm at a loss for what to do.

You can't have it both ways - if you're not out to people about being non-monogamous, and you flirt with or pursue other people in front of them, they will assume that something untoward or inappropriate is going on. One could argue that visibly flirting/pursuing others is essentially outing yourself.

If people are not open minded and accepting enough to understand non-monogamy, it's probably not wise to try and flirt with or pursue them. If you're just pursuing other people in front of them, like giving a cute bartender your number while out drinking with them, then they are probably going to be uncomfortable about witnessing what they think is immoral/cheating behavior.

It sounds like you really want to be able to live openly in your community of peers - so it might be best to come out to them. You could do it more formally, sitting down with them and telling them, or you could work with your partner to bring it up more casually, conversationally, just make it known. Secrets and ambiguity make people uncomfortable; if you two talk and joke openly about how you're ok with your partner seeing other people, that usually works better for everyone else.

When it comes to the specific person you're trying to date, you need to be really clear with them! Just knowing that your partner knows you're openly flirting isn't enough. If you want to really make a go of trying to date them, you need to let them know exactly what's going on, how you feel, how your partner feels, what they would (and wouldn't) be getting involved with by dating you. If you don't feel like you can have that conversation, then you should probably let go of pursuing this person.