Window Security Certificate for calendar not even in use but can't find how to delete it

Use the reminder/hidden-items cleanup approach, as the prompt is coming from a calendar subscription or reminder that is no longer visible in the Calendar list.

Follow these steps on the affected Outlook profile:

1. Close Outlook.
2. Clean up hidden reminder items:
   1. Select Start, type:

            outlook.exe /cleanreminders
      

      and press Enter.
   2. Let Outlook start, wait a minute, then close Outlook again.
3. If the prompt still appears, remove the corrupted reminder directly using MFCMAPI:
   1. In Outlook, select Send/Receive > Work Offline, then close Outlook.
   2. Download MFCMAPI from the GitHub “Latest release” link.
   3. Run MFCMapi.exe.
   4. On the Session menu, select Logon and Display Store Table and choose the mail profile.
   5. Right‑click the mailbox store and select Open Store.
   6. Expand Root – Mailbox > Finder, then double‑click Reminders.
   7. Sort by Subject or To and locate the entry that corresponds to the boss’s calendar/ICS URL.
   8. Right‑click that item and select Delete Message.
   9. In Delete Item, choose a permanent deletion option and select OK.
   10. Close MFCMAPI.
   11. Select Start, type:

              outlook.exe /cleanreminders
       

       and press Enter to restart Outlook and rebuild reminders.

This removes the hidden reminder or subscription item that is repeatedly triggering the Windows Security prompt, without needing the calendar to be visible in the Calendar pane.

References:

• You can't remove a calendar item or an email message in Outlook in Microsoft 365
• This calendar isn't available error when a user tries to access the Calendar in Outlook on the web