Sorry, but I am not up on what materials reflect what wavelengths. A thermal camera detects long wave IR and that does not matter if it comes from a reflection. That is, if you look at yourself in the mirror with a thermal camera you will get a weak reflection from the body heat. Other surfaces might give a stronger reflection. That alone tells me it is the surface that is a bigger factor more than the actual material.
As I recall, if gold is buried in dry sand it holds the heat longer. Heat conduction is low. If it's in damp ground it sure seems like it is not going to work as the heat will be drawn away too fast. Even if it's in an insulated container, the amount of heat escaping is going to be so low as to not be able to detect it. I must be wrong here somewhere.
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