You might not see the mice, but can probably hear them, mostly at night, as this is when they tend to be more active.  Your pet may even paw at walls and other areas where mice might be hiding.  Look for mouse droppings and nests in the kitchen, garage, basement and other storage areas.  Mice will often stock pile their food.  One client found acrons underneath the bed covers in their guest bedroom.  How do you control mice?