Thancred almost completely disappears around soon after you kill ifrit somewhere around lvl25ish or so, that's when he is sent to investigate Lahabrea. And that's it, there only couple of mentions about to his sudden disappearance and that's that. Mind you the quest where he is revealed is around lvl 45 ish. Getting from 25ish to 45ish takes a long time especially if you're not playing it every day. Meaning you can completely forget that Thancred even existed. Because you don't even do a quest to figure out his wheareabouts. He is literally forgotten for 20 or so levels. And then you make a sudden reveal without ANY prior explanation that such thing could be possible. Oh wait i'm sorry there's like a sentence told by Minfilia that such thing could happen. She told it either before she got captured by the empire, or after the reveal had happened i don't remember it well enough. So please by all means explain that