If you go to the south-eastern "lighthouse" at the end of the harbour promontory, you are given a quest to clear out the robbers on the chain of islands to the north of the main island. However, when you return for your reward there isn't one, and the "quest" isn't referred to again.
Returning to that torch, I wonder if it's just a distraction. It may be there just to draw your attention so that when you turn to light it, a collection of rogues thieves and vagabonds can race up the passage behind you and stab you in the back. What do you think?