Grand jury is so named because traditionally it has greater number of jurors than trial jury, called a petit jury (from the French word petit meaning small).
Although mice tend to come out in the night when most of the occupants of the house are asleep or immobile. This allows them to stealthily infiltrate any of your food bags, crumbs etc. During the day time, mice will normally not come out, but if opportunities exist then they surely can.
Mice will sneak inside if they can. If mice get into your home they can cause all kinds of problems, including destruction of your property by chewing on things and by spreading disease as they leave urine and feces everywhere. In the garden or on your patio they nest in your plants and damage your plants.