She's just not that into you. Sorry to borrow or co-opt a term from a cheesy chick series, but it's true. If you have to guess or rationalize the reasons why or why not, it's not a clear enough signal of interest. 99 times out of 100, when you're getting "mixed signals" from a girl, it's because you're being overly optimistic about the "yes" part of the supposedly mixed signal. Chicks aren't that subtle in displaying romantic interest, trust me. When a chick is genuinely interested in you, it'll be as plain as day. If I were in your shoes, I would most certainly not risk the workplace awkwardness (and maybe even worse?) that would ensue if you asked her out and got shot down.