Of course that's true that they will need proof of all your metals. Believe me or not, till date I have a habit of buying these metals with the receipts. I take special precaution to preserve the receipts along with the metal. This will happen in future that government may try to find such things to boost their revenue.
Even Cryptos will be taxed one way or the other, like while purchasing because once you purchase it is not traceable. Though they may have sophisticated model to some how tax Cryptos. We may see this coming when BTC goes mainstream. Yes Cryptos are decentralized but government may try to bring it under their scope by governing the exchanges or having strict laws for trade within the country. ICO is one of the example. Regulation may start flowing henceforth.