An Observation based on a recent scan

[Audo]

Yes, she could.
Xemnas believed the RoA was in CO before Larxene or Roxas or Xion even joined. If he had found it, then he could have easily found Xion there later.

 

[DarkSoldier85]

...I'm pretty sure I already made a thread exactly like this not too long ago.

And in that thread, we said that Nobodies must be born in in-between worlds (Twilight Town, The World that Never Was, Castle Oblivion). The Room of Sleep is in Hollow Bastion, a world more affiliated with darkness.
Edit: Beat me to it Audo. But I'm pretty sure HB/RG is a dark world (cuz of the DTD).

xion couldn't have been born in the RoA because thats the room that xemnas is looking for. and since xemnas goes to the nobody and names them, that would have meant that he had already found the RoA.

Look at the scene between Xigbar and Zexion in KH2FM. He mentions Marluxia JUST JOINING. So Xemnas may have found the room between then and when Xion joined.

 

[Audo]

Radiant Gardens is a light realm world. >.>