Rodents of all shapes and sizes (mice, schrews, and voles to name a few), have little to no trouble accessing the inner workings of buildings no matter how well a building may be "sealed up." If there are doors, there is access for rodents.  Whether it be an industrial, commercial, or residential building, there is the potential for these rodents to take over. Mice begin reproducing as early as 6 weeks of age and after a period of 3 months, what was just two mice, can multiply into 20 mice and just one of those mice can lay up 50 droppings per day! Indoors, there are no natural predators (except us) to control the rodent population and you may soon have a mess on your hands.