Hey, Bungie has acknowledged that Skeleton Keys are going missing. You can see this here, there is no ETA on a fix: https://www.bungie.net/Mobi/en/About/News?id=45613&transparent=true[quote] • Skeleton Keys may be removed erroneously from Character Inventory[/quote]From what the community has observed, this only happens when a key is delivered to your Postmaster and is not picked up for awhile. The key in the Postmaster seems to delete itself along with all other keys collected on that character.