Bungie has made it clear that not all content in the Vault of Glass has been found.
Recently It was shown that there is another passage to the Templar challenge, just underneath where the first chest is located. Most think this is a jumping challenge of sorts but someone is not so sure. The area is vast, and has a lot of nooks and crannies to explore yet seems completely unused.
Now, going by how the 5th chest needing certain goals to be met before appearing, perhaps this area is where the 6th chest is meant to be, once triggered?