The problem is that govt officials occasionally make up their own rules and sometimes they get away with it, when people acquiesce. I used to deal with govt in my business, and sometimes they would ask for things that were not required by the regulations. I had to remind them that they couldn't just make up their own rules, and there were rules that they also needed to follow.Correct! In India they know all about it. And likewise with life in "the land of the free..."
Any good law enforcement agent will be able to tell you that PM is not the same as cash. And they are not going to argue the point, and that's the end of the matter. Even bubblebobble2 was not fully prepared for what awaited him, but he knows better now, for he is better informed as a result of what life has thrown at him.
Riddle me this... which are these two individuals is likely to have their metals seized by law enforcement: (a) the prepared, or (b) the unprepared?
Essentially you should be carrying paper proof that you have purchased "like" metals (not necessarily the metal you are carrying); e.g. one oz gold coins. You probably won't need it, but if you do, you're prepared.
The naive would whine, "oh but I should be able to carry my metals around freely without any proof of ownership." When the wise-man hears this, he maintains a discreet silence, but the ignoramus is in full agreement with the naive man, hence he rarely ever hears any contrary points of view, and so he goes along blithely, until one day his metals are taken from him by the authorities.
So the law is "you're innocent until proven guilty". It's not "you're guilty until you can prove your innocence". So if you have cash you don't need to prove it's legally yours just because an official thought you had too much. They can ask you questions about where the money came from, but it's up to you as to how you answer. I think my first question would probably be, why are you asking this question, and what does it have to do with airline safety?