Maintaining the System Storage is critical task in AS/400.
There should be some Scheduled jobs which performs Purge process and Housekeeping activities of Server.
You will need to check the Jobs which ran after Sunday morning.
This process doesnt need to delete any objects. It will save the backup of the data into archive libraries into tape and will clear the Old Data(Eg. Keeping the Data for the past 3 years & Purging the previous data).