The item weighed 1 pound and I wasn't moving at all.
Well then the guard's helmet must have been enchanted with Detect Life. :P
Wait... What?
The Unmarked Cave was apparently part of a quest that didn't make it to the final product, a quest that may have involved a Red Queen and a Black Queen, judging by that rumor and the "Black Queen's Hall" zone of the Unmarked Cave.
The rumor of the Red Queen must have been left in the game by mistake. It's so puzzling.