London has an old school gold exchange. Pretty much you can buy and sell by moving a part between vaults (at least that's how I understood it to work in the past). Now I expect it to be just a spreadsheet but who knows.
Because instead of treating each item as an individual thing, they were pooling everything with other items of the same type. So if I send in a brick and say it’s an iPhone 17 and you order an iPhone 17, you might get my brick.
I've no idea what checks are done to exclude such artifacts from gold storage. If somebody accepted them and stacked them in a reserve vault, they might go decades without discovery.
I guess it's not something to worry about. It's improbable, and even so, if everybody agrees on the value of undetected bogus W/Au ingots, makes no difference from the real thing.
Its audits only look at a tiny percentage of the holdings at a time, any kind of weighing or purity inspection is the exception rather than the rule, and third-party auditors or observers are not welcome. They basically look at a handful of bars, see that the seals on the other vaults are intact, and extrapolate that all the gold is perfectly fine.
In other words: the US could be pulling the fraud of the century here, and we'd never find out. But a full audit would be extremely expensive, there's no sign that there's any kind of foul play, and other countries are doing more-or-less the same.
And even if a country does notice that something is off with their US holdings, there's plenty of reason to keep it quiet and just play along.