Or perhaps they fell out of someone’s pocket while they were emptying the trash. As far as remaining in perfect condition - gold does not oxidize or otherwise corrode. When gold coins are recovered from wrecks they sometimes have coral or shellfish attached to them or accreting them together, but other than that they are generally unaffected by their environment.
People who “empty the trash” do not have 13 escudos on them. That was an ENORMOUS amount of money at the time - more than a year’s wages for most folks.
Wouldn’t 13 gold coins have been a huge fortune at that time, the life savings of a merchant, and unlikely to have been dropped accidentally, or even carried out of the house?