You say that no other full backup was performed.
But if the server is backed up using some backup software solutions, that can be seen by SQL Server as a backup.
We use a network backup software that includes an agent on the servers being backed up. When backups are configured, we can set rules to include SQL Server and/or Oracle databases as part of the backup set.
If we do that, SQL Server is aware of the backups and resets the differentials.
Is it possible your environment is doing something similar?
If you run this query (modified to exclude log backups):
CONVERT(CHAR(100), SERVERPROPERTY('Servername')) AS Server,
WHEN 'D' THEN 'Database'
WHEN 'L' THEN 'Log'
END AS backup_type,
msdb.dbo.backupset.name AS backupset_name,
INNER JOIN msdb.dbo.backupset ON msdb.dbo.backupmediafamily.media_set_id = msdb.dbo.backupset.media_set_id
WHERE (CONVERT(datetime, msdb.dbo.backupset.backup_start_date, 102) >= GETDATE() - 7)
and msdb..backupset.type <> 'L'
You will get some data that can help confirm or deny this possibility. You want to watch these columns for clues:
- Backup start dates that do not line up wit your backup job's schedules.
- Physical Device Name column entries that aren't the path you use in your scheduled backup jobs.
- Backupset Names that aren't what your backup jobs set.
On point 2, for example, I use the backup jobs from Ola Hallengren. In my environment, I use them to back up to a server share. So that column in the results looks something like this for the database "DB" (with Full or Diff depending on backup type):
But in our environment, the CommVault backup software agent creates device names that are GUIDs like 1c20c18e-6f55-4bc5-86d3-5db7f9eca5d9
On point 3, the Ola H. scripts do not set a backupset name column. But the CommVault software generates this backupset name: CommVault Galaxy Backup.
As an aside, if you aren't using the Ola H. backup scripts, I highly recommend them.