Is there any record of it hitting the mail server at all?
This is probably diving into paranoia, but could it be possible that the user is setting up a rule to divert/block/deny delivery somehow, but since Outlook 'sent' it, it shows up in the sent items?
Or, the user makes a contact with their manager's name (but fake email address), they 'send' their report, then delete the contact and any bounceback they get?
I don't know why someone would go through this effort just to avoid sending a report they seemed like they were going to send anyway, just thinking outside the box.
Plus I like devious shit like this :)