Okay, now I am the one looking for a fic...
It's a small one, less than 1k words, I think. It is told from the pov of Stiles' stalker, who observes him from afar while Stiles is in a diner with the pack (?). The stalker is obsessed with him, goes in detail about Stiles' features, and is convinced that Stiles likes him. Then Stiles leaves the diner, and the stalker goes after him. The stalker doesn't realise that Stiles noticed him, and thinks that Stiles is unusually skittish. Then Stiles hides in some grocery store, and the stalker sees him calling someone while distressed. After a while, he sees Derek (he doesn't know it's Derek) coming in the Camaro and Stiles running toward him in relief.
In the end, this stalker watches as Derek pulls Stiles to himself and I vividly remember the stalker's shock at their familiarity. Like, this dude honestly believed he and Stiles would end up together, and that Stiles was his already. His internal monologue goes something like "He could just imagine Stiles under that man, moaning and how dare he". Then, Derek, with his arm around Stiles, turns to look at the stalker and says "Mine", and that's the end of the fic.
I remember it in such detail, and it was very short, and we were never given the stalker' name. I cannot find it no matter what I try.
I humbly request your help, sterek comrades 💗