So I got an idea. It’s quick, it’s dirty, and I’m not entirely certain it’s possible. It’s possible for monsters to spawn after x number of days. Is it possible for after y number of days a percentage chance of a “horde” spawning after a notice. For example it’s day 80 and you get a message.
“Something feels wrong, like there’s trouble on the horizon.”
then a day or so later
“The local zombies seem more active than usual.”
another day or so later
“You’re noticing a lot more zombies in the distance than usual.”
and finally
“You see a massive horde of zombies coming! Better dig in!”
at which point the game will continuously spawn zeds just inside the reality bubble for z period of time, possibly aggravated by noise?
It’s not perfect and frankly it’s another “patch” flavor but there you have it.