I had mice for thirteen years, in a new house. Everyone had the same comments about “you'll never figure out how they are getting in, they get in holes as small as a dime,” and “get a cat” (my dog wouldn't abide this) and “I'll exterminate them with poison and come back whenever you need me to”… I had to get a new stove, because the old one stank with mice. All our stored items in the basement were marked with signs of the mice. It was so depressing not to own our space.
Then finally I found people who were smarter than the mice!! Batguys.com came and they looked around. Matt climbed into the window well and he spotted little grease signs on the window and a bit of insulation–little signs that that was where the mice were entering. He identified one small breach in the otherwise well-made house. He blocked it off, we trapped the several mice who were thus trapped in the house and alas, a year later, still no sign of the mice. I have my house back! Kudos to the Batguys.