Apparently, the course_completed event is an event that occurs in the cron and not in the direct interaction of the platform with some user. So it is not an event that can be observed.
That's what I could find.
Could you find a solution?
If not, would you have to do it using the task API? observing if there is a new user with the complete course without being flagged in my module?
Sorry about my English, I'm Spanish-speaking.