Many mice never enter your home. Especially when you have performed extensive pest exclusion.
Our native Deer and White-footed mice (Peromyscus) are very versatile and excellent climbers often living high above ground. Sometimes this includes our attics. New England Wildlife: Habitat, Natural History and Distribution cites 25-30 mice per hectare (2.471 acres) in some test site areas, male Deer Mice have been recorded having a home range of 2.3 acres! Non-native House Mice populations have been documented as high as 300 per acre, the odds are they will find your home eventually and settle in.
This little fella spent the night on my porch. Two days later I found him (or a relative) in my gas grill. I think they like my caramelized Barbecue Sauce.
For information on getting rid of mice, contact EHS Pest.