Where did Quina go in Madain Sari?

Okay, I absolutely love Final Fantasy IX and its story. It's pretty darn solid overall. But there seems to be some kind of plot hole. When the party goes to Madain Sari, Quina is there and s/he helps Eiko cook up a feast for everyone. Afterwards, s/he goes to sit by the river side.

Then, after the party goes to the Iifa Tree, Quina just completely disappears from the ruined city. I didn't find any dialogue or Active Time Events which explained this at all. S/he is just gone. Then, on Disc 3, s/he suddenly appears again in Treno while you're supposed to be winning the card game tournament.

What? Is there any explanation as to what happened here? I'm hoping that I missed something, because this would represent a glaring deficiency in an otherwise awesome and seamless story.

The Wiki entry for Quina says:

After the party leaves the Outer Continent leaving Quina behind, s/he swims away from Madain Sari and, with no real explanation, ends up in Treno.

Which leads me to believe that it is never mentioned. I don't remember it ever being explained in-game either.