Zombie Player Tracking without ability to see them

Zombies in a dungeon grinder are tracking players they cannot see. The Zombies in our spawner resist the water and ice used to funnel them into a water column to go after the player that hit a zombie last. We have a solid wall between the player and the zombies, however all of the zombies in the area( including in the caves) will try to track the player that most recently hit a zombie.