she's likely an attention seeker and wants everyone to see it cause it makes her feel good about herself but at the same time she'll make you out to be the creeper if you say anything so this'll go a couple of ways for you.
one she'll be all "um ok" and tell all her friends how you're staring at her ass crack and how you're obviously obsessed with her "weirdo giggle giggle" or she'll be all "mind your own perv"
either way ignore it or stare (which is likely what she truly wants anyway) but don't say a word to her.
No, I don't think so she probably knows it is hanging out, and she probably wants it to hang out if she is that kind of girl. I knew a girls who would jack her thong up over her pants so guys could see.