On March 15, around 5 pmish EDT, my group went to fight Karkadann. When our WAR went to proc the red !!, which was Raiden Thrust. As soon as the red !! disappeared, so did Karkadann. We sat around for a few minutes, unsure what happened. We checked to see if the ??? had reappeared, but it did not. After a few minutes, we went to fight Rakshas to get another pop. Around 10 minutes after he disappeared, out of nowhere, Karkadann appears and nibbles on our WHM (The person who had been tanking him had warped back to Jeuno at the time.).
I understand that he's based on Dark Ixion, but I'm not sure that Karkadann is supposed to just flee like that and come back after several minutes when a normal group would've moved elsewhere.
The only other note I know of is that it was Lightningday, which we only noted because it was a thunder-based weapon skill against a thunder-based mob.