I was playing one time on split screen with my friend we were just getting started and we were on round 11 or something then no zombies attacked us or were in sight but when we went to the crumpled wall bit there were like 6 zombies just standing there.
And once it was pretty much the same thing apart the zombies were inside by the crumpled wall and they were just standing there but when we got close they started running
But if we didn't move forward they just stood there for 2 mins then they would come after us (this was about round 14) and this happened quite a couple of times