California-2007 Drone Craft

In April, my wife and I were on a walk when we noticed a very large, very strange "craft" in the sky. My wife took a picture with her cell phone camera. A few days later a friend (and neighbor) lent me his camera and came with me to take photos of this "craft". We found it and took a number of very clear photos. The craft is almost completely silent and moves very smoothly. It usually moves slowly until it decides to take off. Then it moves VERY quickly and is out of sight in the blink of an eye. MORE THAN ANYTHING I simply want to understand what this is and why it is here?