1) So, just because she doesn't respond aggressively to the 'accusation', which, by the way, is probably meant as a joke - she must be gay? Nice logic.
2) Some people are flirty. I'm a lesbian, and I flirt with the majority of my male friends. It's easier to flirt with guys because it means nothing. It's all for fun. They know I'm gay and it's fun to banter like that. So maybe she's straight and she likes to flirt with girls because it's easier, too.
3) That's you're personal interpretation of 'the look'. Obviously, if you believe she might be gay and hitting on you, you will see things that will enforce your belief. 'The look' could be nothing.
4) She could be joking. I always say I wish Silky was my boyfriend because he is very kind, sweet, and buys me chocolate every time I see him. I don't fancy him, it's just that I mean it would be good if he was my boyfriend because he is fun and he treats me well. Maybe she sees you as a good friend. Or maybe she's that particular kind of flirty person.
Explanations for every point you had, but if it's really bothering you, just begin to probe her about it subtly - for example, say 'have you ever done anything with a girl?' If she is the flirty type, then it is likely she will easily answer this question. In fact, it's almost certain. Then you can just go from there.