Some POIs have fixed zombies. One of the army camps for example will always have a group of radiated soldier zombies in the loot room no matter what gamestage you're at (to be fair that one is a T4 POI though)Pretty sure there are more. I set my difficulty up, and thought it was that, but talking to some other friends that play, they confirmed ferals are now present in POIs from day one, even on default difficulty.