I second your concrete plug option if you find a hole, Mike.
No sense messing around, fill-in any holes you find permanently. Not only will a proper concrete plug keep the mice out, it will keep other creepy-crawlers out as well.
It's almost impossible to seal up a house to keep mice out unless you make it air tight.
There will always be crack somewhere. Wall, foundation, window well, etc.
They can squeeze into a crack as small as the width of a pencil.
Around here mice a part of the eco system. I live near farm land and next to a forest preserve.
There are birds of prey, coyotes, foxes, snakes, etc. that all live on eating small rodents.
We have mice, field mice, voles. ground squirrels, rabbits, cats, small dogs, that all part of the food chain for those predictors.
One acre of farm land can support 5000 voles.
If a mouse wants to get in your house, they will.
Traps are the only things that work.
When I stored my hot rods, I had all the anti-mouse stuff I could use because they will destroy the interior of a car so fast.
Irish spring, etc., in a house will just make them go to another part of the house.