Events with custom eventtype automatically deleted

I'm using the WS core_calendar_create_calendar_events API call, and setting the eventtype to a custom string of my choosing as I want to be able to automatically manage these events separately (they describe teaching topics for each week of each course).
Everything works fine, the events are created.
 
But every hour or so, they get automatically deleted by something!
 
There's nothing in the Scheduled Tasks log to say anything is running, but my events keep getting 'cleaned up'.
 
Any idea how to find what might be doing it?

Thanks!
Jules.