Dying of radiation because you wandered through too many craters? lol.
Being stalked because it caught your scent? Yep.
Being stalked because you enjoy listening to noise emission FM[sup]TM[/sup]? Yepyep.
Having something spawn nearby in the middle of nowhere and drawing a horde from hammerspace just because the RNG is being a bitch? No.
I’m still in favour of the general idea, but they need to be static. That way if they wander into you or your scent trail and follow you around it’s legit. That said making them hard to detect would be good, maybe a chance determined by perception and survival (aka tracking) you get something like ‘you feel like you’re being followed,’ or ‘you feel undead eyes upon you!’ or something like that. Maybe we could add a Paranoia trait that allows you to detect tracking easier.
That said If you’ve been encamped in your fortress of doom for the last few weeks never to see the sun, there really shouldn’t be these things tracking you. As for them making a better trail to follow you by, it sounds pretty good. How about making them moan every few turns or something like a shrieker, except zombies that hear it join the horde for X turns reset to X turns on the next moan if in range. So, as it passes through populated areas, it draws a larger horde.
As to general toughness, I’m thinking of 2 types: first type – Tank. Slow~Medium speed. Doesn’t need to be fast because of tracking stealth. Damage sponge, medium~high hp, pretty good damage resistance, low/standard damage output. Emphasizes the horde calling strategy.
second type-- Killer type. fast (something like zombie dog or brute speed) medium hp, average damage resist, high damage output.
Emphasizes the ninja zombie assasination stuff.