most likely the bullion banks have it. Central banks routinely lease their gold out to privately controlled bullion banks. This is one of the way that they can inflatethe amount of gold that actually exists. Because the physical gold is on the books of the central bank and the bullion bank at the same time. Theoretically the bullion banks use the gold for investments then return it to the central bank.
However it's a very real possibility that many of the bullion banks simply The gold and never returned
JP Morgan owns the largest privately controlled bullion bank if I am not mistaken