If you use the BCC option and send a PM, if you view the PM in the outbox you will see the recipients and the BCC recipients.

But if you go to edit the PM, the BCC recipients list is not displayed.

Having looked at the posting_pm_header.html template, under the 

<!-- BEGIN bcc_recipient >

 section I see there is an 

<! IF not S_EDIT_POST -->

 if statement for the delete button, so I am assuming the BCC list was originally intended to be displayed when editing PM's.

If this is a bug, it is probably caused by the following if statement: 

<!-- IF S_ALLOW_MASS_PM -->

 which doesn't seem to work on the edit PM page.