It is possible that it would be in a maintenance plan. It is possible that it is being run through a 3rd party app. It is possible that there is a sql agent job running it. It is also possible that another person just run a tsql command for a 1-time backup.
Here is a script that can help you get a history on the backups
http://jasonbrimhall.info/2012/11/30/backup-history/
Jason...AKA CirqueDeSQLeil
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I have given a name to my pain...MCM SQL Server, MVP
SQL RNNR
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