He found the ship's books; but, owing to his ignorance of Spanish, he was unable to read them.
The name on the stern of the vessel was St.Jago, therefore he reasoned that it must be a West Indian vessel.

How the idea entered his mind, Stevens never knew.

It came suddenly, as an inspiration, that the galleon must be a Spanish treasure ship.

One day, while in the captain's cabin, he found a narrow door opening from it.

It was securely locked, and though he searched everywhere for keys and found many, none would fit the lock.