Already posted this but apparently I posted it to the wrong place so here goes :
In this fic, Derek is a police officer and he finds Stiles sleepwalking with no shoes on. Stiles has some kind of colour coded classification card on him so Derek checks it. (can’t remember the details of that bit) Scott comes to his rescue with shoes and a promise that he’ll watch him more carefully. Derek is convinced that Stiles is on drugs and so he mentions something to one of the deputies at the station who promises that Stiles is already being looked after. Then at some point, Derek finds out that Stiles is the sheriffs son because Stiles has some kind of seizure in front of them both. Thanks!
FOUND IT!
https://www.archiveofourown.org/works/21037679/chapters/53331997#workskin