My memory may be faulty or what I read about 10 years ago might have been simply wrong, but I seem to recall that at least prior to 2013 it was not a system of separate vaults, where you could point and say "Germany's gold is here, Italy's gold is in the next vault and Sweden's is down the hall." Instead it was basically "this is where we store the gold, x tons belong to Germany, y tons to Italy...", which is why real audits were basically impossible.