Same problem here. i'm in an complete empty area, suddenly 3 zombies spawn in my view about 10-15m away and start to engage me. I shoot them down and suddenly 3 more are spawning at the same location. i went up some stairs close by to get a better shot, but out of nothing there is a mob of 20-30 zombies running towards me. after i killed those too, i'm going out of the building and 3 zombies are spawning at the same position as first mentioned. after i killed those and some more i'm running out of ammo and die. if loads of zombies keep spawning in front of you there is really now way to survive :)
