After all emails in the inbox have been marked as 'read', the number next to the Inbox (as shown below) does not decrease or disappear.
Applies to: Outlook 2013
Select the Click here to view more on Microsoft Exchange link at the bottom of the list of emails, as shown below.
This will show the unread emails the number is referring to.
To remove the number next to the 'Inbox' folder without finding the unread emails, you can right click the Inbox and select Mark All as Read.
Alternatively try the fix below.
1. Type read:no in the Search Current Mailbox (Ctrl + E);
2. Click the Find More on Server link.
The unread emails should then show in the list.