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If the Organization and Ansem weren't able to find it, it must have specific conditions to so, such as being a Keyblade Master or having Ventus' heart. It's most likely located on Floor 14 of Castle Oblivion.
The requirement to find the hidden Chamber is being the master of the Land of Departure, which Aqua currently is, and wielding the Master Keeper/Master's Defender, which Aqua currently has.
As she was the one who formed Castle Oblivion, only she knows the location of Ven and is the only one who can find him.
Where does Floor 14 even come from?