Here's the scoop straight from Rumour Control.
My understanding is that many years ago they used to do the physical move of gold from cage to cage and re-mark the bars. pure gold being rather soft, it was fairly easy to "erase" Lower Slobia proof/ownership marks and put Upper Volta's on. Then they stopped moving the gold simply becuase they were just moving it back and forth every other day. The more times you touch something the greater the chance of a screw-up.
So what happens is it's all pretty much in one big pile in one big cage, and they just keep track of it upstairs either in a ledger or on a computer.
Which is all good and well, now at this point you'll need your aluminum or tin foil beanie (gold would work also).
Okay so you get this up and comer, China, and they put gold in the vault. They wait a few years and ask for some back, then some more, and later a bit more. So they shipped back whatever easiest to grab and load. It's all gold, right?
Wrong. It appears that some of the bars may not be 99.9999% pure gold. Not even close. So certain .govs find out that some/many/most/a few bars may be salted. So what do they do. The smart thing: "I want MY gold back." If they are your bars, with your markings then why melt them down and re-pour them? Only if you suspect that you might have Iron Pyrite (Tungsten really, as it's the closest in weight to Au).
Now if you do have funny money, you really can't call them out, because 1) you don't have any real proof it was them, 2) Triggering the 2nd Great Depression after what you did the prior century will not win you many/any friends. So you just kindly point out that "Hey, the Cold War's been over from 20+ years now, I really don't see Russia being able to get to Berlin without Ukraine and Poland at least slowing them down, if not stopping them, Plus their in NATO, so Uncle Sugar can save our bacon. This Time.
Oh, and then throw in the Nazi angle where maybe some of the former conquered countries are wanting back what was taken in War II. There's always that.
So that's the poop from rumour control. I've got to get back, I need another layer of foil.