If you live in a semi or terraced house, then they can also come in that way. Not sure how that can be stopped. Keep all food unable to be attractive to mice in mouseproof containers. I had very few mice when I had cats. One was chased up the stairs by my cat and the mouse disappeared under the skirting board by flattening itself. The cat sat there for ages, waiting for it to reappear. That is the only mouse I ever saw there. Also, if someone is doing building work nearby, that can cause mice to appear in your place.