You need to buy a good UV flashlight asap so that you can see where that thing, or things, have been. The UV will highlight urine stains and travel paths. You will be surprised when you turn the lights out and shine that UV light into an area where the critters frequent. This may also help you to solve the mystery of where they came in, just remember that a Rat can fit through a whole the size of a quarter or less depending on the size. If you have a cat or dog, they would be targeting the food. Also, usually when you see one, you have six.............keep at it until you are sure that you have absolute eradicated them little "insert bad words here" rodents.