Stench

Zombies have an acute sense of smell. In both combat situations and laboratory tests, they have been able to distinguish the smell of living prey above all others.

In many cases and given ideal wind conditions, zombies have been known to smell fresh corpses from a distance of more than a mile. Again, this does not mean that they have a greater sense of smell than humans.

Put simply; they have to rely on their sense of smell more than we do.