That a guy gets stared a lot by another guy?

I am mystified by this social nuance as well! I think a lot of people do this, "people watch" because they are interested in a person's personality, or want to find out something about them (especially if you have a poker face). I do this as well, but I'd like to think I'm a little smoother about it than the guy you described. I have recently caught this one girl staring at me in class one day, but for my case I think it's just cause she was interested (not sexually).
But all I can say is do you. Yeah its awkward when ppl you don't really know are openly "studying" you without any inclination of their motives, but besides confronting (saying "hey" and catching them might not be a bad idea either..) .I would just say try to put it off as they are just curious.
And unlike alot of these comments, it may not be anything sexual as I have explained. Good luck!