Definitely, definitely flirting. That said, if you're poly* and don't know whether she is or not, probably best to, at the least, let her make any further moves. Of course, my dating advice is best taken with a heaping teaspoon of salt.
*I'm assuming this from your using the word 'primary' up there.
You assume correctly. I'm letting her make the blatant moves, because I don't want to make things weird. If she's that interested in me, then she'll bring it up. I do wonder if she is put off by the fact that I am married. I did recently post an article about poly on Facebook, so she may put two and two together. But then, there's also the fact that she wouldn't want to be a secondary.
I actually suspect this will turn out to be ships passing in the night. She seems pretty cool, and I don't want to jeopardize a friendship just because I want to jump her. If it happens, it happens.
The reason for the doubt is that up until then, I haven't seen any interest from her. Sometimes you can just tell based on what the person is doing. For example, finding excuses to sit nearby. I had one girl who said, "Didn't you notice I was coming to see your show?" Okay, that was more arrogance on my part as an actor than a fuckbuddy. I thought she wanted to see the show. Heh.
This girl just didn't send out those vibes until the hat incident. Made me raise an eyebrow. But then again, actors are really, really friendly and don't quite have the boundaries like normal people ("We are actors; we're the opposite of people!").
I'll just keep on trucking and leave her opportunities to approach me. I also don't want to be the creepy old guy trying to mack on younger girls (even though she's out of college).