I've been having this issue on my work iMac for months now. The Reminders app will launch, pop up a notification for a split second, then disappear before I can see what it was trying to notify me of. This happens when I don't have any meetings or appointments scheduled and as far as I can tell they're for ghost events. Meetings/Appointments that I do have with reminders have reminders that work as expected (pop up and remain until dismissed) but not these ghost ones. I'm getting 4-10 of these a day and I can't figure out any sort of pattern other than they tend to appear on the 15 & 45 minute marks of the hour (makes sense for 15 minute default reminders).

Does anyone have any idea what these are and how to get rid of them?