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.
Tree squirrels are active during the day -- unlike most other rodents such as mice, rats, raccoons and opossums. Being mammals, they like nice temperatures, so you find them during the summer time, during cool hours -- early morning and late evening.