Had another thought, and I know you guys said no aliens but....

This was literally revealed as a mystery that even Meteion didn't know, we couldn't really deal with finding it out at that time since we had more pressing matters but...

Nekropolis. They just vanished, gone, no trace of who they were and why they died. That could be a mystery in of itself of what happened, especially if it started happening to other stars or was a threat to ours.