My theory, is that when Xemnas was talking to Aquas armour, Aquas armour talked back... That is why the other Org members heard someone speaking to Xemnas, this could possibly be Aqua. Now, Aqua must have thought that she was talking to Terra, as Xemnas is partly Terra, so she may have told him. That explains how he knew Ven was there. Why he was looking for Ven? Possibly because Ven was a keyblade wielder and Xemnas wanted to use him to collect hearts for Kingdom Hearts. This is just my theory though, it may or not be true, but it sounds right.