The purpose of the Missing Drive is to prevent documents from being removed from a device if a cloud drive is accidentally unlinked.
The "Missing Drive" is added to the Drives section of the Documents view when a Cloud Drive, like DropBox or OneDrive, is unlinked. This drive contains all documents that were downloaded to the device before the cloud drive was unlinked.
Missing Drive documents are not synced and remain on the device until they are either deleted by the pilot or the pilot signs out of their ForeFlight account. When the Missing Drive is empty, it is removed from the Drives section.