Well, let's start speculating, then...
It's 200 years later. The original plan was to get people off of earth. So when the argo didn't return, maybe they went on and started the exodus anyways.
Getting the whole population off in 200 years... Nah, that sounds stretchier than the "Alien" hypothesis. Also, we know that Clarke picked up a message which he thought critical enough to delay arrival by 200 years. So no... the late arrival cannot be the cause, since the whatever happened seems to be the cause of the late arrival. It is also something that wasn't entirely unexpected, judging by the conversation between Clarke and Adams, but dramatic enough to withhold it from the crew.
So that's four major clues, all in all: No people, no bodies, not entirely unexpected, and something you don't want to end up in the middle of. Don't know, I can't make anything out of that yet. The suggested massive solar flare does not sound too implausible, except for no visible remains whatsoever.