This reminded me of my sibling. When she was a teen she decided to breed exotic pet mice for money. She got some siamese, long hair, etc., and set them up in cages in the garage. Before she could start breeding them to each other, she found the females were all pregnant.
It appears that wild mice can fit through the mouse cage wires that domestic mice cannot. Those boys from the wrong side of the tracks were sneaking in, having some fun, and then taking off.
She never could keep them away from the girls. They managed to get through every type of containment. If there was an airhole for her mouse, those boys got in and out.