"Unknown Accounts" appear within report filters when there are costs associated to AWS accounts that have been deleted from the AWS console. Deleting an account on the AWS side will result in that account, and all assets included within that account, to no longer exist. While the platform will have information associated to the accounts and assets provided in the billing file, none of those assets will be visible in other parts of the platform.
Cost drilldowns will expose the assets associated to the Unknown Account charges but none of the detail will track back within Asset or Account reports. These account charges were most likely linked to a consolidated account that still exists (and is referenced in the drilldowns) so the data can be linked back to the account.