IMO, it was a military test involving a probably sub-launched missile.
It seems clear that the military is keeping the exact nature of the test secret, but I would speculate that it involved the Global Missile Defense system in some way (there's an installation at Vandenberg), perhaps also some sort of ship-based missile defense, and likely some space-based assets were involved as well.
As an aside ... it could just be a new way of launching military missions into space. One wonders if the Sea Launch platform is still tied to the dock... Seems to me that such a ship would make a dandy platform for launching polar missions, and the location is such that it could leave port in Long Beach and reach the launch site in a matter of hours; and then come back after dark.
A US test sounds plausible, but why would they do it so close to our own coast that its visible to land based civilians? Why not take it out to sea farther so no one is watching?