The Non-Owner Mailbox Access Report in the Exchange Administration Center (EAC) lists the mailboxes that have been accessed by someone other than the person who owns the mailbox.
Run a non-owner mailbox access report
Note: When a mailbox is accessed by a non-owner, Microsoft Exchange logs information about this action in a mailbox audit log that's stored as an email message in a hidden folder in the mailbox being audited. Entries from this log are displayed as search results and include a list of mailboxes accessed by a non-owner, who accessed the mailbox and when, the actions performed by the non-owner, and whether the action was successful.
References: https://technet.microsoft.com/en-us/library/jj150575(v=exchg.150).aspx