Are you using a Symantec/Veritas product to backup the server? If so, it likely recognizes that there is a SQL Server on the system and is using the SQL Server methods to back up the databases (so that they are good backups). The Agent the product installs on said server typically runs as NT Authority\System (Local System if you look in Services). That would be my guess as to where the backups are coming from. And likely then you don't want to disable it without talking to your backup administrator.