I think the zombies are learning.
Yes, some of them start out smart, like the marked one and the ones who can talk. But I think the
others are capable of learning, at least to some degree. For example, in the
army base the zombies know that
Angel can't see them. The jumper knows that they are in the other building. If you look at the first lot of zombies in teh army base the first time, they were really stupid. "We left the keys in the door". It just seems like, as the series progresses, the zombies are getting smarter.
Also, the leader may have been a zombie for a while. Remember in the first episode, they mention how the mental asylum is only about 10 miles away. That's a big distance but not when your as fast as they are. We know that a marked serial killer was put in the mental asylum because he was judged insane. What if he had already been a zombie? As far as I can remember, they never mention exactly how he killed the people.