It usually a way of flirting when a girl says it to a guy, but I don't think it means much of anything when a guys says it. Haha, they're not that great at flirting.
It means "Hey, I might not remember your name..." or they are trying too hard to be chill
No I'm sorry that doesn't mean much. However what comes after that may.
It means he doesn't remember your name . . .
Sometime I say "Hey you" to my girl with a lot of 'familiarity' overtone when I saw her or if she called and I recognized her voice. She would replied with a flat and relaxed "Hey". I did not meticulously plan it, that's beyond my acting abilities, but it just came out that way and suddenly there was this nice and intimate aura around us. Very nice. And then we would carry on fighting again - Just kidding.
No, it does not mean anything. Other than a "What's up?"