when you keep flushing toilet paper (USD) down it.
Im not sure if there would be some register for the serial codes or what would happen with the gold if you didnt hand it in. I'm not sure if the bullion has the year it was minted or something like that. Might make it harder to sell through reputable channels. I am of the opinion it wont happen here but the westernised world will follow suit in one form or another if the US decides to price fix, tax it or similar. They did pay currency for the confiscated gold btw.
