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.
The activity levels of the mice will increase when tested at night, when the lights are off, when compared to during the day when the lights are on. Activity level is defined by number of rotations the mice do in a 10-minute period.