We have occasionally had mice in the attic area and (less often) basement, the entire 12 years we have lived in this 1990 house. The first winter I killed 32, but only a few a year since then. Despite the death toll, I still get more every year. I have looked everywhere and have sealed any possible entry sites. A possible big one 10 years ago was a poorly sealed soffit under a cantilevered bay window, with only vinyl siding spanning the joists. Despite covering that with plywood and sealing it well, we still get mice.
I went above the aluminum ceiling over the mud room entrance and saw the remnants of a small acorn dinner party. I have seen other such shell scraps in the attic, but have never known if they are from the time period when the house was under construction, or are much newer. Once I have removed them, I do not seem to have them recur. I do not know where any gaps are large enough to let in acorns.
However, I now realize that the space between the bricks and this aluminum ceiling has gaps large enough mice can squeeze through them. I have access to both sides of this ceiling. What should I use to try to seal this? An image shows the gaps from below. I had not been concerned by them, but they look more impressive from above, especially at the mortar.