But it also affects everybody who was in the original casting individually. So if a new person walked into the city, Xykon could scry them, but since Haley and company were in the city at the time of casting, they're covered.
One thing worth noting is that it's only living creatures who get targeted individually by the Cloister, so once they're outside its effects, Vaarsuvius should be able to find Roy's corpse using Discern Location, assuming s/he happens to cast it with that intention.
