Ok. So if he used a gold coin it may have raised some genetic eyebrows. But if he never spent a gold coin then probably they were not raised.
I go with the latter.
Now, he was a British volunteer (to become "freed") and did serve through operations in New York. Perhaps his modest wealth came from a share of the looting or something that was a war trophy (not shared around)? That WAS a common practice at the time. Doesn't even need to be all thay mysterious top require Templars, burials, vaults, or even chests. And maybe something not so proud to admit afterwards.
Why make it fantastic? Much more simple answers work. Or, just plain hard work and good business sense/investments/luck could explain it as well.