When a user manually adds a contact and is waiting for subscription approval, Spark creates the Unfiled group in the roster. It goes away when the subscription is approved on both ends, but if one of those users goes offline while another is still online and then goes back online, then for the user who still was online Unfiled group will show up. Although this group is always empty as currently there is no way to add a contact without a group. Maybe there is no need at all to have Unfiled group now as pending contacts are shown in the groups they were created in.
Sometimes Unfiled group can show up even when no other contact is online (after many user additions and removals). Unfiled group also temporarily appears when removing manually added contact.