A person who stalks you so much that they know where you are and will be at all times. So in an attempt to not be creepy, they will guess (correctly) where you will be and show up before you get there. This gives them the false feeling that they are not creepy when in all reality you understand they are only there to see you.
-"Man I think shes pre-stalking me."
-"Shes been following me alot and last night she showed up at the atmosphere concert when everyone knows she only likes opera."
-"haha man, sucks for you"