At NWAF, I noticed that if I aggro a zombie while on top of one of the hills (top of hangars, those weird plumbing system hills?, etc.) , the zombie will chase after me, and instead of chasing me up the hill, they dissapear, and go through the hill, and are now under the hill terain.
From there the zombies are stuck there, and keep making noises. This happened with almost any hill I went to, and I just heard Zombies underneath the ground?
So yeah, Infected are able to go through hills, not run up/climb it, and get stuck under hills.