They have had a few shows on TV about Fort Knox and the gold it contains... One man who worked there on a summer job some years ago said the place was full... Another show I saw more recently shows how years ago the Fort was clearly visible for some distance and any trucks going in to move out the gold would be clearly visible... Today they say the place is over grown with trees and shrubs that make it impossible to keep track of what goes in and out... The show also made a point that people lurking outside the fence are cleared out quickly and are not allowed to take photos... I believe some senators went in years ago and the fort opened one of the vaults so they could view some of the gold.
I don't know this for a fact but I suspect that allot of the gold is "double counted, once stored", that is it is counted as US gold reserve because it is stored in Fort knox, but it is also counted as part of Germany's gold reserve, and France and Italy and some other countries that built up their reserves in the 70s without physically allocating it to their territory. Also some of it is pledged to the IMF and the World Bank, but I am not sure that the amount is written off the US reserve, so again- one gold bar might be counted multiple time and have multiple owners.
And also some of the gold that is supposed to be stored in the vaults might have been lent away since the senators visited the vaults. Otherwise I don't understand why when the Germans have recently asked to ship their gold to Germany they were told it would take something like 10 years to ship only a fraction of what their gold which is supposed to be stored in the US.