I've noticed this issue a few times since .55 came out and got some confirmation from friends. Zombies appear to spawn after you have been in an area for a few minutes. There have been several times where after exiting a building there will be between 1-4 zombies in the immediate vicinity (within ~50 yards) that were not there when you entered. I don't believe they followed me as they aren't aggro'ed on anything, they're doing their normal walking around and a few would have been impossible to miss on my way in.

Seems to be related to running to a new town so it might be a symptom of lag where the server doesnt register i am in a new area until a minute or so has passed.