I can't understand him ?? ( I think he's weird !!!)?

So I know a guy who's my guy friend brother , we r neighbors and we all go to the same school .. So I cought him starring at me nd when I look at him he looks away , we 've never talked to each other and once I was going out from home then I saw him going out from his home with a girl but we made an eyecontact for 5 secs but his face showed that he was surprised , puzzled and paniced to see me.. So why he's acting like this?? HELP CAUSE IAM TRYING TO UNDERSTAND HIM !! And I feel like that Iam being watched from him and I hate that feelling , and I cant ask my friend (his brother) cause they dont tell each others about their own things... Help me plz , thanks !