How are documents changed to completed in the document list?

The course dates are scheduled or completed based on the ‘Completed Date’ on the course master of the course date eg: a value of -2 will change the courses from scheduled to completed two days before the course date start. 

This is run as a service called ‘course expiry’ - which you can disable if you wish, this means that to remove the documents from the list of scheduled courses you would need to manually change the courses from scheduled to completed.