After I lost my refrigerator to a rat who ate through my condenser, and I had to buy a brand new one, I was unable to find where they were getting in so I had to figure out a way to keep them from destroying anything else. I found these devices that emit a sound that you can't hear and it won't bother pets, but rats and mice hate it so they stay away....I was surprised to find out that they REALLY work! I plugged one in my kitchen and one just outside of the kitchen and I have not had a single problem since. They were only about $11 at the hardware store.
Here is a pic that I grabbed from the Internet of one of the many devices available to purchase: