I know one on earth which involved a ?? Knight hidden in a cliff. You had to lure it to the edge then jump away and hope that it fell, and inside was a chest(hidden usually in ceiling rafters or just in a corner) and a dead ghost just sat in the small to to the left at eye level. The cliff area is near the huge hive spire/crashed ship thing and from there to the area where all the fallen spawn out of a cave near the wrecked helicopters. Hope this helps! :)
Edited by subj3ct18: 7/22/2014 7:26:16 PMi know of one of the ghosts at devils lair where you get to the tank and go into the building on the left and up the stairs there is one that room. and one just as you inter the vanguard room at the tower and its on a table. those are the ones i can think of on the top of my head.