I have a user whose Outlook shows 5 messages in outbox in the sidebar but when go to outbox it is empty. I ran updates, quick repair, changed the download messages to all, ran scanpst, turned off conversation mode and it still shows there are 5 to be sent except it is empty no messages needed to be sent. He travels a lot and noticed it recently when 4 but went to 5 the other day and let me know when he returned back.
As you run scanpst, i assume he is not using Exchange storage and email is stored locally in pst? I think even when running with Exchange we had similar issue with Outlook 2013 sometimes, but it was instead showing unread messages in Inbox. Only deleting and letting Outlook recreate ost files (local cache) fixed the issue for us (temporarily). This issue went away after we have switched to Office 2016 (Office 365 ProPlus) and Office 365 Exchange Online (was Exchange 2013 before).