Not actually creeping up (ie) behind someone, but could also be depending on the given situation.
To know enough about a person that you don't actually know (ie where this person works). Creeping would consist of using that small bit of knowledge you have about said person and going on the popular website called facebook and attempting to look them up.
Katie: Yeah he was super-cute!
Amy: You know right? I just totally went on facebook and looked up the Lovin' Lavish&Squalor group, found a picture of the guy (which was tagged) and creeped his profile. He's super sexy!