So I'm playing in explore mode by myself and I get the public event where you have to take you the three extraction crews. Still by myself I proceed to clear out the first two and start in on the third. The third extraction crew proves a bit more difficult and I'm extremely low on health and about to die when a new player joins the event just in time. He unleashes his super just as I'm convinced I'm going to die and fail the public event! This kills off the remaining few enemies and we end up getting a silver! It was a great feeling.

But if you get a silver that means you only took out the first two crews, doesn't it?

I'm not sure, seemed like he finished off the last crew and the time wasn't yet up. I think the medals are based on time as well as wether or not you complete the objective.

I had a great one last night...

I was in explore mode over near where you go for the strike mission, specifically after the shortcut in the top of the wall near where there are opposing baddies shooting at each other. There was no event announced, but all of a sudden about 10 waves of baddies start coming out of this door. Each wave got harder and harder, and one of the last waves included a level 9 wizard. Luckily some more people showed up and helped, but it was pretty amazing to be nearly overwhelmed after having waltzed around the level for an hour or two with no trouble at all. I wonder if someone else didn't activate it somehow, but I have no idea because there was no in-game message before or after.

Edit: Just found a topic about in bungie forums