already confirmed that the keys are so you can replay the 32, 34 and 35 challenges for loot. what you said about the 28 possibly dropping the keys could very well be true
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Proof that's what the keys are for? And not some YouTube but an actual Bungie statement.
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planet destiny has an article about it. the type of proof you desire will only be found when you see it tomorrow, but it's true. there's a treasure room with small chests that contain stuff like armor cores, motes and coins. then there is one locked chest that requires a key. apparently you will get the keys when you complete the story tomorrow as well. just check it out. planet destiny doesn't release false information. not a day before it happens