"Rodger Whitlock" wrote
snip
My guess is that either they crawl under the door (perhaps 1/4"
clearance) or up the outside of the building and in through an
open window. Since the outside of the building is rough cedar and
shingles, this isn't impossible.
snip
Comments?

Reducing the gap under a door to 1/4" or less is the textbook
recommendation for proofing against mice. They wriggle under doors etc.
by flattening their ribcages, BTW. I've seen film footage this and of a
mouse scaling a 20' vertical brick wall in remarkably quick time.

They are also very adaptable to new environments. There is a fascinating
account of a colony which established itself in a very large freezer store
(in London, England, I believe). Within a very few generations and
living solely on frozen produce with no free water present, the mice had
adapted to their surroundings. They nested in the packing material and
grew longer and denser coats than normal. (The infestation didn't say
much about the vigilance of the staff who worked at the freezer store,
however).


- Tom.