It seems that chests do not spawn in random locations. For instance, there is a chest that spawns every time in Loch Modan in a sunken rowboat.
I need at least 2 moss agates (for a normal scope and for my silver plated shotgun next level), and according to Thottbot it seems the greatest chance to get one is in alliance chests in the Stonetalon Mountains. Also, what are the average monster levels there?