Hi all, I just found a secret golden treasure chest in the scenario accessible by using the Key to the Palace of Lei Shen. The golden chest is hidden inside of one of the banner-like structures so you are unable to see it, yet are still able to click on it. I found it one on of my alts and in the next three runs it was there, so there's a decent chance that it is always up. The golden chest can be found in the second room of the instance, right after you pass through the trap-filled hallway.
On your immediate right you are able to jump off of the ledge of the stairway and down to a lower level where you can find the golden chest inside of the banner. Just click it and the loot is yours! Be careful about the patrolling Zandalari though, some sleep dust may help you open the chest without being attacked by them. Enjoy!