In my earlier answer, I speculated as to how a large quantity of treasure wound up in a dragon's lair. The dragon doesn't just rob everyone and every institution in a provence in a short period of time, right? One anwer is that someone put it there in a large organized effort, and that has a profound meaning as to the question.
Another way is that it was gathered slowly over time. That makes the question vacuous, as it did not vanish all at once.
The talk about soldiers and rowers made me think of another answer. It wasn't taken from circulation but is a fresh resource. Soldiers were famously paid in salt, and that is the basis of the word salary. What if the dragon lives in an exposed salt dome? Maybe dragons seek them out and excavate them, so the cave isn't filled with treasure but carved into it: a salt mine. That is why dragons are associated with caves of treasure.
As a plot device that would be quite a twist for someone to finally discover, after salt is no longer valuable in his society, or the legends have crossed into a region that supplies salt and it's common place there.
The dragons find a salt cave to be great shelter from preditors because nothing else can live there. The need to eat salt to produce metalic sodium via enzimatic pathways, which is what makes them spit fire. A venom gland produces alkili metal nanoparticles surrounded by an oily membrane, suspended like milk. That is mixed with modified saliva which contains an enzime that destroys the membrane, so he shoots boiling steam that's also extremely caustic to flesh and releases hydrogen and methane gas which burns too!
Enzymes also concentrate all sorts of metals from the dragon's diet. Some are essential for catylists. Others are poop. Local ores of copper, gold, or silver cause concentrations to build in in the lair as a side effect of the dragon's need for platnum and and rare earths, and the eating of ore-bearing rocks to obtain it.
Metals are concentrated by symbiotic bacteria— decendent from the kind that helped make geological veins of metal— in a modified auxilary stomach chamber.
So even without the association with salt, you could have a lair build up concentrations of precious metal flakes over a long history of occupation.