Have you exhausted all efforts with regard to plugging the holes? No holes, no mice.
Maybe try using a non-chemical dye of some sort that is preferably washable to allow the mice to show you where they are coming from & going to. You'd need to place it somewhere they are likely to walk through and hopefully somewhere already near where you expect an entrance to be.
His company developed a fluorescent powder that could help you track
mice back to their nests. "When the mice walk on the powder, they get
it on their feet and it leaves a footprint," Stern says. To get the
powder onto the mice, load it into a box with food or dust it onto
cotton balls, which the mice nab as nesting material. Then follow the
footprints, which appear under UV, to find where the mice have set up
shop. This lets you set up traps there, or seal off an outside entry
point if mice are entering into the house from outdoors.
Also related: http://www.mousetraps.org.uk/mouse-traps/luminous-dust-mouse-tracking-kit
Using a sieve, sprinkle a small quantity of our luminous dust around
the area you know them to frequent; where you have seen them, signs of
nibbling, droppings etc.
The following morning and in low light conditions, switch on our black
light torch (included with the kit). You'll be amazed to see how you
can now follow their every action; their footprints will light up like
Follow their previous night's movements for up to 20 metres; find out
how they got in and what parts of the house they visited.
With some luck and determination, tactics like this should lead you to the entrance(s) and you can plug them for good. Even if you eliminate all the existing mice, if there are still holes, there will be future visitors.