There are many situations in which you can't see a zombie, because as we know, being on the North or West sides of walls, you can only see a short distance, and could easily run into a swarm. My idea is that if you can see the zombie, then a yellow outline should appear, so you can actually know it's there without a complete graphics overhaul, whether behind a fence, tree, or wall (behind being relative given the isometric viewpoint.