What triggers non-owner MB access?

What triggers non-owner MB access?

We have an alert set up so that when a non-owner accesses certain mailboxes, the alert is sent. What constitutes this access? Is it when they check the person's calendar, or... ?  I ask because the alerts we're getting are for people who have no access whatsoever to the mailbox (in permissions) -- so how could they be accessing it?

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