Disk shaped craft appeared, slowly zigzaged across town and took off.

I was sitting with group of friends on the back deck of a house in the hills above Fairfax, California.  We'd had a barbeque and were quite laid back watching the end of the day over the valley.  The weather was clear, the sky darkening and our view of the town and valley below was perfect as the house was near the top of the ridge. The disk shaped craft seemed to materialize from out of nowhere. It was not there and then it was.  It was silver in color, but seemed nearly translucent.  The craft, which was approximately 30-50 feet across, widened from the narrow edges towards the middle.  As we were looking down on the disk, we could see a dome on top which appeared to have view ports or windows of some sort.  There were also green, red and white lights encircling the perimeter of the craft.  There also appeared to be a smaller dome on the bottom of the craft. The craft then began moving in a slow zigzag pattern back and forth over Fairfax for about a minute.  It then took off faster than I have ever seen anything move.  It shot straight up and was moving so fast that it seemed to leave a trail of light images behind it that then disappeared.  All of this occured with no sound by the craft at all. We immediately phoned Hamilton Air Force Base to check if they had possibly seen this craft on radar.  We finally got through to the tower at the base and they stated they had not picked up the unknown craft on radar.  I'll give them the benefit of the doubt as the craft was below the ridge line and appeared and disappeared so rapidly that it could have done it between sweeps of the radar.