You just need to be close to the ground (sneaking). The chest is always there, it's a merchant inventory chest. If you can't find that one, the
This chest belongs to Ahkari the khajiit. She was just outside of Dawnstar too. i just happened to watch this shortly after i bought from her
Just go to Dawnstar and look around the rocks to the east/left of Iron-Breaker mine. The chest is invisible, but your crosshairs will see it.
Go to the entrance of Dawnstar, wait 48 hours, and talk to the cat lady and ask her what she has for sale. Then go back to the chest, get your stuff, wait
An Elder Scrolls 5: Skyrim player shows new players where they can go in Dawnstar to find a secret invisible chest full of loot.