Bright white disk, stationery for ten minutes, and then in an instant disappeared completely.I was at the San Leandro Marina and stopped walking to watch a FedEx plane land at nearby Oakland Airport. Then my eye caught an extremely bright white disk across the San Francisco Bay. It just hung in the sky for almot ten minutes, then lighted up to a brighter white glow. Then in an instant disappeared. Completely gone. This should have been visible from the control towers at Oakland and San Francisco airports. I don't know what it was, but it definitely was not an airplane or blimp.