Might as well raid the ones in Markarth, Whiterun and Solitude as well.
Small correction: It's not actually a "hidden" chest in game terms, it's a merchant chest belonging to one of the Khajiit caravans, supposed to be unreachable to the player but Bethesda didn't lower it deep enough beneath the gameworld.
SpacemanSpiff's linked video shows exactly where it is.