It's hardly stockpiled. Most Pt is consumed as it is produced. Although we did have a small surplus last year, it is generally accepted that all of the inventoried Pt the world over would cover maybe 6 months of production. I would have to guess that meaningful inventories exist in Switzerland, South Africa, Japan and the former states of the USSR.
Pt investment fundamentals based upon scarcity indicate a superiority to gold and silver. Obviously scarcity alone won't register in markets, otherwise Pt would be almost 20,000 USD per ounce. The introduction of Pt ETFS have not removed enough supply from market to have any real lasting impact on the price. You could make a similar analysis for Pd, except that Russia is pretty much the whole story in terms of inventories.