From my understanding (as a cop's daughter), an officer can arrest someone off-duty if necessary. However, in a non-violent crime, it's more likely he would call an on-duty officer and report what he witnessed. Also, since Chase is a detective he routinely wears a suit rather than a uniform. If he just got off work, it's likely that he would still have handcuffs in his pocket. When my dad worked in the detective division , he would come home with all kinds of stuff in his pockets. The only thing he would leave at the station was his gun and holster. I can't count how many times my brother, sister and I decided to play with his handcuffs and, inevitably, someone got locked up! Good thing Dad kept several keys around to get us out!