If the necromancer can plan ahead, he can book a place or a cabin or hotel where it's near places that could have dead bodies but just not buried. Like a beach, there's bound to be someone who drowned. Or a cliff or a hill or any type of high ground, someone could have fallen off. A place with snow, maybe someone was buried under it. And the necromancer knows that there people die daily from these kind of places, so he picks places where it is most likely to find a dead body.