I will start this post by acknowledging that dogs do have legendary scent abilities. I have seen a documentary on a dog who when presented with several small cardboard tubes that people had breathed through was immediately and confidently able to find the one that had been breathed through by someone with a specific form of cancer. Most days the dog would get it 100% right, but some days he did no better than chance. Then there are those incredible seizure dogs who are able to smell the chemistry change in a person and warn them of an impending seizure.
Now having said that, I am not convinced that a dog is capable of sniffing out precious metals.
One of the reasons precious metals are precious is their molecular stability and resistance to oxidation and chemical interactions. I doubt there are enough gold or silver based aromatics in the air for even a dog to smell.
Here is a fun fact - you know how copper and brass has a mushroomy metallic smell when you handle it? It's not the metal you are smelling, it is a chemical named Oct-1-en-3-one, which is a by-product of the chemical reaction of skin lipid peroxides and metal ions. The metal is a catalyst that turns your skin oils into a product that coincidentally smells metallic.
I would put it to you that while a dog could (and obviously has, in this case) find reactive metals like steel or zinc (as found in brass) on smell alone, gold and platinum that has not recently been handled by a human would be beyond the capabilities for a dig to sniff out.
Silver may be possible though.
I would love to be proven wrong, and look forward to seeing you factor this into your experiments