We have the same problem in our (tiny) yard in SF. I was HORRIFIED when I saw a rat sauntering around in the middle of the day. Freaked out and called the pest guy, who says there is a colony (!!!!!!) of rats living in an old stone wall at the bottom of our garden.
Jan, I'll be interested to know how you get rid of your unwanted visitors. I have two kids and a very curious puppy, so am scared of using poison, but I WILL NOT HAVE RATS! (and that's final)