I studied body language as part of a college course and whilst most assume a girl touching or playing with her hair is them flirting it is actually a lot of the time a subconscious self comforting gesture, showing that they are feeling shy/ self concious/ nervous, if the girl looks at you holds eye contact for longer than a few seconds looks away looks back and smiles she is flirting, this however sounds more like a show of feeling self concious
It might have to do with feeling self-conscious. If I notice someone looking at me, I might brush my hair aside because 1) I'm worried about whether I look okay; or 2) I'm thinking, "Ack, someone's looking at me! Quick, act natural!" and don't really know what to do with myself.
The adjusting their hair could have two reasons 1) nervous reaction - you make them self concious or 2) they're showing off their hair to flirt. The hair adjusting could be totally unconscious but - unconscious action or not - looking back at you is definitely them flirting... well, probably perhaps not definitely.