The chests are what Bethesda uses to store merchant's items. They have nothing to do with treasure hunts.
Here are the places I know of. Finding the exact details of how to get to them will be up to you.
One in Dawnstar near the mine. It's invisible, already gone over in this thread.
One in Riverwood trader, requires a wooden plate and a Do Not Delete to get to.
Three in the market area of Windhelm. Requires a follower. You get to them by glitching through a wall, and commanding your follower to loot the chests from the outside.