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.
Any kind of clutter gives rodents cover to travel in and out at will. Use a black light to detect rat urine. The average rat urinates 300 times every night. Each drop will show up under black light (much like some other substances that police investigators use black light to detect in crime season investigation). Where you see rat urine, you have rats.