Rodents are very dependent on their sense of smell due to their poor vision. Because of this they leave grease/scent trails and small spots of urine and feces everywhere they travel. This allows them to navigate quickly without being solely dependent on their sight. The more these trails are used, the stronger the smell. Even after the rodents are removed their scent trails remain. This allows new rodents to follow the old rodent’s trails. If you do not properly exclude your house from rodent entry the cycle will continue without end.