They aren't. You have to set their persistence in the CK.
Guards and other levelled npcs dont need persistence. Unique actors do.
You put a vendor in a corner of a city, he uses the location of that corner for persistence.
If he have a boss LocRefType, it would point to his persistence location, in theory.
However, if you place him at his home, and his home is in another corner of the city, the location of his home will be used by the one where he works, according to the CK.
My question was if this would affect placement of quests (for instance, courier quests), in case he is not to be found at his working place, when you pick up the quest.