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For anyone still stuck (SPOILERS):

That metal fragment is in the cave after all.  It has nothing to do with light in-game as far as I can tell.  The guide text file says it's hanging from the ceiling there, but I still had trouble finding it.  It turns out it's at the end of one of the hanging chains.  Because it's metal and rounded, it looks like part of the chain, so it's really easy to miss.  You have to look closely to see it.  If you hover the mouse over it, it will change to a yellow hand cursor.