If you ever played a true fantasy tabletop RPG, you know it. Zombies are everywhere. And no, it doesn't mean you can find a necromancer in all those places. Zombies don't need them to lead their un-lives. It seems they're quite independent creatures, doing absolutely fine on their own. Whatever they're doing.
There are many leftover places where you can find restless dead, animated by some strange force, an ancient curse, some magic item, a cleric or other magic user. But do not fear to disturb them - they won't notice, they're already disturbed.