Images at the bottom.
In VOG after the gorgons lair there is the zone with the teleporting platforms you have to jump down on. Right at the beginning before jumping down, if you look to your left you will see a cave. Going through the cave takes you to a dead end opening looking down to the doorway to the last boss. From here if you look down and to your right you will see a bunch of ledges far down. Jump down to them be careful not to die. Down here is a small purple cave that doesnt seem to have much use. Weird part is once you are down here teleporting platforms start to appear.
These teleporting platforms lead to the boss doorway but you can also use them to get to the broken vex teleporting ring in the distance. The recently discovered 5th chest is on one of these and has special requirements for it to spawn. Im guessing if there is a new chest you have to complete the gorgon cave without getting killed once or maybe even kill a gorgon.
Also at the beginning of the raid before you open the door. If you jump off the ledge in the middle where the middle pad is to defend there is a vex teleported that looks exactly the same that doesn't do anything not sure if that's important.
http://imgur.com/VzGKgxG
http://imgur.com/VWBlzuD
http://imgur.com/8IdmfX3
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After someone beats and the raid an gets all 5 chests they should go to the raid in orbit and check to see if the rewards say you can still get raid gear. That would mean there is more chests to open.