In Tryton, just before you enter the Upper Boughs (you are outside, on the tree-fort stuff) there is a torch that is not lit. It's in a corner, somewhere along a walkway.
You can try to use items on it, but, so far as I can tell, nothing activates it. I went back, just before I beat the game, and tried using all the different items I had. No luck.

Anyone figured it out?

I don't think anybody's found an item that you can use there. File it under unfinished quests. Maybe the fuel for the burner is locked inside the condemned house in Arnika...

That was supposed to be activated with insense, i've heard, but it's an unfinished item, so you can't activate it.