What is psychologically wrong with this person?

there is this person I know that hangs out with a bunch of ppl we'll call him Greg. Greg has very odd behavior at times, he is fairly nice guy but is considered very odd. Sometimes when at someones house you will see him peeking around going into rooms that he has no reason being in and when asked why he is there makes up some dumb excuse however he seems innocent enough, like you don't want to get mad at him cause he jsut seems a little off you know, but he mostly is nosey and really quiet about it. Also has been known to sleep at his friends bedroom door however, claimed he was drunk and ped out cause it was a party that night but he hadn't drank much only like 2 beers, he just does some really odd things and when confronted on them seems really confused it's really weird. There is definitely something wrong with him I am just wondering wht it could be. He also loves doing things alone like going to concerts and well basically places that most ppl would want to go with others