For sure! I once watched one get into my basement through a gap in the
mortar between blocks. The mortar seam was a -bit- wider than normal
but not by much. I would have sworn there was no way for the skull to
pass through but he went through like he was greased.
I have heard it said that if a mouse can get his nose into something up to
his ears, he can get his whole body through. As you said, I have seen them
get through some very small holes.
And if you've ever wrecked out a house, taken off drywall and exposed
studwork, toeplates, and top plates, you know there are holes in there big
enough to stick your thumb through with plenty of clearance. Lots of extra
holes drilled by workmen for pipes, vents, drains, conduit, and such that
were never backfilled.
Lots of room for a slick little mouse.