(again the textures are just stand-ins because I had them from digitalizing LNOE awhile back"
The green capsules are zombies. The green lines are their current paths. Many are just random shuffling, if they don't sense or hear something in rnage they pick a random location up to their maximum walking distance and head there.
The longer ones that all terminate in the red capsule are the zombies that have sensed a nearby hero and are converging on him.
All the red dots are blocked nav mesh points that have to be pathed around.
They have a zombieHunger variable which is how far away they can sense the heroes. There's also a zombieHearing variable for listening. My thought on the last is that when ever there's a fight, if a player runs or searches a location, a noise marker is dropped. The 'loudness' of the marker is how far away a zombie can hear it. So searching a location, fairly quiet, meleeing a zombie, a little louder, using a firearm, pretty loud, using a chainsaw very loud, using a stick of TNT pulls every zombie from across the board. They're only pulled of couse if they dont' have a Hero targeted.
So each round the players make noise would start to pull zombies in from various distances across the board depending on how loud it was. This could force the heroes to constantly have to flee their scavenge points.
It could also open using something loud in one section of the map to pull undead toward that area to give a couple of turns of non-interrupted searching in another spot. And allow for 'loot cards' like "Alarm clock" "Walkie Talkie" etc to give the heroes resources to manage the zombies.