Sometimes when zombies appear, they emerge in places where they trap themselves. It seems that they are programmed to appear on the spot outside the building that is closest to the active sim. When my sims are in Shang Simla and a full moon occurs while my sims are at the base camp, zombies will often appear on the patch of ground just behind the kitchen.
The trouble is that the zombie can only move between a few squares of land and will get frustrated quickly with a route fail like this. The only way to fix it, short of editing the lot to provide an escape route from this location, is to reset the zombie sim which teleports him back to his home lot.