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 are indeed nocturnal little beings, according to the University of Kentucky College of Agriculture's website. This is especially the case when it comes to wild mice. Pet mice that reside with people in a home setting may engage in activity both during daytime and nighttime, however.