Bless Unleashed

Private treasure chests appear on the minimap as a treasure chest icon. They give a small amount of resources and a bag piece, 4 of which can be turned into a bag merchant for another inventory slot. I unlocked this in the 3rd region of the game, which isn't even half-way through the map really. There doesn't appear to be a map out yet with stuff like chest locations and such, but if you keep an eye on the minimap as you do the story, you should have this well before the story is done.