First go to door and put a platter or wooden plate up to it
Then push into it to get through the wall
You will fall in water(move back if you just keep falling)
Then you can enter the 6 hidden chests and loot them!
These respawn after 48 hours if you are Guildmaster but only 1 box respawns if you are not (Thieves Guild Fence chest) thats the best one anyway!Click here to view the video