Describe reason for admin signing in as a user

When an admin uses the "log in as user"/"sign in as user" feature in the web admin interface, a confirmation prompt appears warning the admin that all other admins will get notified by email. It would be nice if this confirmation prompt included a text entry box where the admin could describe the reason for signing in as a member, and if this reason was included in the email sent to other admins and in the audit logs.

Use case: Our admins are regularly accessing the Dropbox account of a recently departed senior employee to look for certain files. Currently, the Dropbox admins manually send emails to the other admins to let them know the reason for the access.