The world is unknown, filled with ruins, strange creatures, and far flung human settlements.
Strange lights are sometimes seen at night, perhaps these are monsters, but perhaps they are secretive humans sailing past.
In a world where people must cooperate to survive and thrive, why would someone sneak past friendly communities? If they are bad people, or thieves, why would they have visible lights when passing established villages? If they are good, why would they avoid contact?
There are mysteries, big and small.