Although I'm a Chemist, I'm far from being any kind of expert in gold and/or silver chemistry, however I'll just toss out a few things I thought of while reading this thread:
1. I'm a skeptic by nature and tend to not believe things unless I see them, so my first tendancy (and I suppose that's the scientist in me also) is to cast doubt on the fact that any caches of gold or silver can give off any kind of noticeable "glow."
2. That said, I do believe it's possible that over time a supply of gold or silver could come in contact with a multitude of different things that could chemically have an affect on the metals. Soils can be acidic or basic, there are definite decomposition products that can occur in the ground that give off other chemicals etc... I can't personally think of any gaseous compounds of silver or gold, but I won't say they can't exist.
3. It seems to me that what's being referred to here is something more along the lines of a "luminscence" effect rather than some kind of gasses that "ignite" or self combust or something to produce a flame. I honestly doubt that there is any kind of real flame occurring when you look at what it would take physically for that to happen - however, the idea that a small portion of the metal has experienced some reaction that allows it to give off a "glow" for a short time (while I still have my doubts) isn't beyond belief imho.
I don't have time tonight to look anything up online, but if anyone wants to research this further, I'd say do some searches using the words "luminescence," "photoluminescence" or "flourescence" along with gold or silver and see if you find anything.
Although I'm a skeptic regarding this idea of caches giving off different colored "glows," depending on the metal present, I'm also not dumb enough to say it can't happen, it just seems to me that it would be a rare occurence and would require an awful lot of perfect conditions for it to happen.