Don't ask a girl if she has a b/f. Women have pride, and the ones that are in fact single may say they aren't. If she's married she'll probably have on a wedding ring or at the very least stop you before you progress.
What may have happened here is you had her FEELING excited when you were talking/flirting with her. And that's why she was orbiting you afterward. She was likely interested, but then her logical brain may have kicked in when the guy kissed her; maybe he sensed she was in to you and was tryna mark his territory.
I'll give you examples from your field report:
Emotional state on:
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she flirts back heavily
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I go for the number and get it very easily, she's still heavily flirting with me.
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Later that night she returns 2x with really bad openers, giving excuses to walk up to me like 'do you know the time?' and then hanging on for 10 minutes to stay with me.
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She keeps flirting and flirting and after work I decide to go hang out with her some.
Out of high emotional state:
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she responded to with 'nooo I just wanted to talk with an old acquaintance',
Chalk it up to an experience but don't read too much in to it with regard to other girls going forward. Assume a girl may be down to play if she is giving you all the right signs.
BTW you said she "vaguely" stated it was her b/f. She MAY have lost her interest when you ignored her after the guy kissed her and then asked her what was up with her and him over text the next day. Or it may actually be her b/f.