Well, after Days, Axel was much more hell-bent on finding Roxas/bringing Roxas back/doing whatever he could not to lose Roxas, possibly because of Xion's betrayal/departure/death/whatever.
The other Members of the Organization don't seem to care much either way so no mentioning from them is understandable (and the lack of Proof of Existence would be explained in Days, probably).
The other people that knew about her were DiZ, Riku and Namine, but DiZ with his whole attitude of "Nobodies suck, boo", Namine's lack of appearance and Riku's plea of silence due to guilt, they'd never say anything either Until the FM++ comes out with new scenes
As for Xion and Riku on DI, well, Xion seems to be looking for Memories, going by her 'Who am I' issue and how Roxas behaved in KH2 - which is no doubt affected by Xion. She could've either came looking for Riku himself and found him there, or came there on her own, searching for Memories of the Girl that Looks Like Her - being Kairi.