If the mice are finding a food/water source on the floor - dog/cat dishes - they won’t climb on to counters/tables etc, where you’d notice droppings. Floor droppings can be missed on rugs or carpeting; we’re not talking cowpies or German Shepherds’ leavings...
LMAO!!!!!!!!!!!!!!!!!!! Yeah, I can tell the difference between a mouse turd and a German Shepherd turd...but thanks ;-) LOL!!!!!!!!!