Having been surprised unpleasantly by zombies falling through the roof from the above floor, I decided to clear out the second story of a house before the first. I then began to explore the first. Somehow, I had missed 15 zombies in one room upstairs, who immediately swarmed me. I went back upstairs to check that room. No hole? Conclusion-Zombies not only teleport, they are master carpenters that like to screw with me by making and fixing holes?