Logging mailbox/calendar activity

I would start by disabling the account on any mobile devices, there have been numerous occasions where different mobile apps have caused various issues with calendars, including deleting items. In general, auditing should cover any deletions, but best check it directly (Search-MailboxAuditLog) as the audit log in the SCC is not very reliable. Just in case, you can also check the Calendar diagnostic logs: https://docs.microsoft.com/en-us/powershell/module/exchange/mailboxes/get-calendardiagnosticlog?view=exchange-ps