In the early stages of an outbreak, people tend to capture, not kill, zombies they have known in mortal life. When the captors have either fled or been devoured, restrained zombies may remain for years, able to repeat the cycle if released. After an area has been swept for ghouls, sweep it again. Then, sweep it again.
Zombies could be anywhere--in sewers, attics, basements, cars, air ducts, crawl spaces, even inside walls or under mounds of debris. Pay particular attention to bodies of water. Zombies wandering at the bottom of lakes, rivers, even reservoirs have been known to surface well after an area has been declared safe.