I know it is public knowledge to a lot of people already, but when an event "fails" to happen, and there are enough people around that it should have, there is often a war, where high level enemies spawn, between two enemy races instead. I have noticed this in Skywatch on Earth and The Barrens on Mars. These wars break out at approximately the event time and no public event occurs. So if you are waiting for a public event and you suddenly see a bunch of high level enemies spawn and start beating the crap out of each other either head for the next public event, if that is all you are looking for, or take part in the mayhem.