As someone who has had a mouse issue before, they typically make holes in the wall near the floor. I have never seen one like the cartoons, but every place I have lived had baseboards around the floor so it is easier to chew through the wall above it. They can’t walk up walls like spiders, and insulation in the walls typically ends near the floor so that is the most likely place to find them scurrying around.

Not sure if older houses were built without baseboards (in which case they would likely make holes at the bottom), or if the curved holes look more like an entrance human-like characters (which cartoon mice like Jerry are typically portrayed as) would make, but it makes sense for a tiny humanoid mouse that walks on his hind legs.

Edit: as for covering up the holes, that is exactly what they did at my old apartment. It was behind the stove so appearances didnt matter, and they just slapped a metal panel over it. I imagine that would be unattractive to have in the middle of the living room though, a constant reminder that you live in a mouse-infested building.

It wasn’t uncommon for arch shaped holes to be cut near the floor to allow plumbing or radiator pipes. These holes were often the perfect size for rodents who used them for easy access into the space behind the wall.

Old wooden floor planks and wooden skirting boards would get crooked and leave big gaps where mice loved to make their nests. The half moon hole shape is just more cartoonish and easier to draw than a one inch gap.

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