It was in the novel. They're discussing DiZ using Oblique Japanese Terms ("that man") and Demyx is like, huh? that man? who? He tries asking like three different people and Xigbar is finally the one who makes him shut up. XD

The scene in Hollow Bastion where Sora meets OXIII for the first time makes me think that Xigbar (the one who comes down to talk to Sora) was not only a friendly figure, but familiar enough with their younger members to the point of learning something about them -- he recognizes one of Sora's expressions as being one that Roxas used a lot.