I’m not terribly familiar with how Cata tracks things that are outside of the ‘reality bubble.’ So this may not be applicable at all, but in abstraction;
I’ve played at least one game where sites the player wasn’t actively in would be tracked with sort of a threat level. Like a scale from 0 to 10+, where 0 means the place is clear and 10 means it is absolutely teaming with enemies.
Each level is a potential amount of enemies. If it were a tabletop RPG it might be a 2d10 roll or something. Whenever your character enters the area it ‘rolls’ the number of enemies in the area.
In order to abstract the movement of large numbers of enemies outside the character’s influence, you would sort of peel off one or more threat levels and move them somewhere else. The actual number of zombies doesn’t matter until the group enters the player’s sphere of influence, at which point the game could figure out how many zombies are in the group.
This accomplishes a few things;
a) It allows the game to track large groups of ‘living’ entities without devoting much processing time. Hordes of zombies could move around the world and it would be akin to tracking the pathing of 30 individual zombies, even though it might represent 300 or more.
b) It gives you some control over how zombies move around rather than relying overly much on their pathing:
- If a ‘site’ can be defined as something as small as a cave or as big as a city, then you could put hard limits on how many ‘levels’ of zombies can fit into that location. A cave might only be threat level 4 while a city might be threat level 10+City Size.
- There could be a subtle long-term pressure by the system to depopulate overly populated areas (the chance of a level of zombies leaving a site is dependent on how many are already there compared to its ceiling), which would mean that you could start the game with prisons having very high threat level (10 of 10 max) and so they would be a natural tendency for prison zombie populations to leak out and join surrounding sites. If this overpopulates that site then the zombies will continue to migrate until the area’s zombie population is relatively homogeneous.
- Certain sites could be ‘locked’ and cannot receive or donate threat levels until some condition has been met. This would allow you to preserve dangerous or ‘easy’ locations until the gates have been opened or a specific amount of time has passed, or perhaps just as a random event. It would be interesting to clear out an area and then have a FEMA camp open up and suddenly three levels of zombies are wandering through town.
c) This would ensure that zombie hordes aren’t spawning from nowhere, which would be frustrating. A well built up part of the map would be more dangerous because there would be more sites to attract threat levels of zombies, but you could, over time, clear out all the nearby sites and buy yourself considerable time before a threat level manages to wander in from far away. So you get the best of both worlds; threat of zombie hordes, but the possibility of clearing an area out. It just takes more time and a wider net than it has before.
d) This would make it possible to have very, very dangerous sites in the beginning that slowly become less dangerous over time, while increasing the danger of the world around it. This is a nice difficulty scaling measure for an entire area, that also opens up some very dangerous specific sites to exploration later in the game, after the population has thinned out a bit. This means you could potentially make access to certain locations (which may have high-tier gear) particularly difficult until later in the game. Another nice balancing mechanism.
e) Potentially you could use this to communicate information to the player about how dangerous a place is. This might not be a great idea depending on your view, but it might be nice if you could scout an area and discover its threat level. Since threat level is kind of a variable anyway and not a specific amount of enemies (if it were indeed a roll of 2d10 then that is 2-20 zombies per threat level, meaning a threat level 1 site could potentially have more zombies than a threat level 9 site if the rolls were wonky enough). But it would let the player know ‘hey, this area is scary’ or ‘this area has been cleared out.’ Add in a delay so that it only updates once every 24 hours or something, and you remove the ‘certainty’ element, so you can’t really be sure an area is cleared until you wait a while. (I have a feeling this wouldn’t be popular, as I’m sure some would prefer a ‘go by what you see’ philosophy, but I thought I’d tack it on there)