Not to be a detractor but I'm with you, there doesn't seem to be any real reason for their to be 7 chests... And to everyone that says 'Bungie likes to use 7's' I can give you plenty more examples of them NOT using the number 7 as you can of them using the number 7 (to those of you playing at home this is known as conformation bias) That said I really do HOPE that there are more hidden chests, I just don't see any evidence for it.