Here is another example of mice getting into beekeeping equipment. They built a nest in this one.
To avoid trouble with mice, store equipment in a pest free place, especially if the hive has died in the winter. Also, if the fall cluster is not big enough to fill two boxes, consider wintering in one box. This will keep out the mice and give bees less space to heat.
If this does happen in your equipment, it needs to be carefully cleaned as mice can spread disease to humans.