Easier way to find shared mailbox permissions

I'm wondering if there's a way to query for what I will call the reverse record for auto-mapping. Outlook dynamically adds shared mailboxes that the user has permission to. Is Outlook brute force searching though all objects looking for permission, or is there a PowerShell command that I can use to find them quickly.

Something like:

get-AutoMappedMailboxes -user john.doe@domain.com

and it would return the list of mailboxes the user has permissions to:

sales
accounting
etc

This would be a cool feature, if not already there!