IIRC, During WWII the Germans continued send to Switzerland gold as payment for it's WWI reparations. Guess were most of that gold came from. Actually, no not the camps. But from the looted vaults of the various central banks of countries they overran. Yes, there was some from the those swallowed up by the Shoah, but some/most of that was was stolen/looted by the various Nazi functionaries before hitting the Reichsbank.
And yes, Waitone is right, it used to be that if one country sold gold to another all they did was move it from one side of the vault to the other (or maybe just on paper even). I suspect that someone somewhere found at least one fake bar with all the right markings and that has created a "Oh, Schiesse!!" moment.
As if things aren't interesting enough...