Okay, I'm going to try to explain this as easy as possible.
Just because a girl "stares" at you, doesn't mean that she's going to hop on top of you when her boyfriend's not looking and make sweet love to you then and there. By no means am I saying that. Pursuing girls that are already taken is a complete and total No-No. I mean come on guys, don't you have any morals anymore? Would you want guys lurking on your gf like a bunch of hungry hobos on a McDonalds bag? Good lord, sometimes I think I'm going crazy whenever I hear stuff like this. There are PLENTY and I mean "PLENTY" of other, free and available girls that you could talk to.
Look at it like this guys. If you thought a girl was interested in you. Say her name's Cassie. She has a bf. Say his name's Brian. She looks at you one day. You go up to her when her bf's out doing something, ask her out, and she says yes. Ends up falling for you, and dumping her bf. Where does that leave you? Yea, you have a girl, but what's going to keep her from having some other shmuck do the same thing to you? See, it all comes around to bite you in the ass. So, no matter how good the bait looks, don't take it. Guys, if there's one area we DJs seemingly always fall short on, it's not looking at the consequences of things. Sometimes the thing that looks "good" to you may not be worth the risk. Think about it.
But, to answer your question, no they're probably not interested. Just because someone glances at you momentarily doesn't mean they want to be with you forever. Stop overanalyzing and getting your hopes up. You probably glance at girls everyday, but does that mean that you want to be with them? NO, probably not. They're just eye candy.
Lesson of the day: You'll get yourself in trouble if you mess with other guy's girlfriends. IT'S NOT WORTH IT
