Ok I found it in the CK.
It's not invisible, it's just underground but close enough to the surface that you can access it. Like I said, it was just a mistake by the developers, who forgot to lower it enough.
if its the merchant's chest, does that mean at some point they stop by to restock?
Things like that aren't actually done by the merchants. The chests refill the same way any other resetting chest in the game does. If it has been altered, it will restock after so many days (2 I think), provided you don't load the cell it is in in that time. When it resets, it empties everything from the chest and then reloads what is supposed to be in the chest. As there are several levelled and randomized lists in the container, it chooses one random item on that list to represent that list, which is why the items change over time.