Scratching or touching nose is usual in uncomfortable situations, I don't know if I can call it flirting because its more like hiding - lying to the person you care about or usually just hiding something, hiding that you like him for example. The best way of knowing the person sees you positive and maybe hides something from you is if he/she is scratching his/her nose, usually the tip of the nose, to be precise.
I remember me scratching my nose while looking down when the girl I liked walked past me. So this part is probably something both men and women have. But while shy men look down, shy girls look just far-far away to still make shure that they are being looked at... Dave, does she play with her earrings or necklace also?