Steps to reproduce:
1. Create an event with a series.
2. Make sure the event is the only in series
3. Delete the event
4. Delete the series
The series still shows up in the media module
The series is gone
Workaround (if any):
It seems like (at least in OC 3.3) series are never removed from the search index when being deleted through the Admin UI, regardless of how many events they contained.