WARNING: The user hasn't logged on to mailbox

Office 365 EDU A2

Recent go-live (December 23, 2013)

I am using Powershell to generate a list of those who have not yet logged on to the system via this command:

Get-mailbox -resultsize unlimited | Get-MailboxStatistics | select displayname, lastlogontime

At a point in the list, an error is generated:

Duck Donald                                           1/2/2014 8:50:15 AM
Mouse Mickey                                              1/9/2014 2:55:44 PM
WARNING: The user hasn't logged on to mailbox 'DiscoverySearchMailbox{D919BA05-46A6-415f-80AD-7E09334BB852}'
('6fb87cfc-249d-4948-9761-ee692af5a1ed'), so there is no data to return. After the user logs on, this warning will no longer appear.
Mouse Minnie                                            1/6/2014 1:49:06 PM
Flintstone Fred                                                 1/9/2014 3:10:30 PM

How do I determine which mailbox is having the problem?  What is the DiscoverySearchMailbox?

Thanks!

         dave