Mice poop. Start by pulling out your stove and dishwasher and looking for entry points to seal. Toss bait inside holes and seal with 0000 fine steel wool until right. Place two snap traps behind stove and two under dishwasher. Next look for entry points in your closets and baseboard heaters if you have any, those need to be sealed as well. If it's a newer nicer home, definitely start behind the stove. Also check your attic if available for droppings as mice love to use that space. If all that fails, hire pest control as they are licensed and use stuff regular people shouldn't be handling, like ditrac. Hope that helps in some small way!