Stuck on the Cactuar Quest

Okay, after finding the one that hides behind the Sphere, I can't find the one that they say is 'stuck' somewhere. . .I heard that he's stuck in a chest-but WHICH ONE? I looked in both sandpits near home, the one of to the side before home. . .no sucess. . .so where is it?

3rd area/map, run as far as you can south then west and there will be a dead end with a chest, it origanally contained 2 mega potions.