Outlook 2003 emails get unfiled

I’m running Exchange 2007, client machine has Outlook 2003. I have a user that files messages dated Dec 3 to Dec 6 in to folders and marks them as ‘read’, but a few days later, the messages reappear in the inbox as ‘unread’ as if he never filed them away. This happened 3 times. I don’t know what or how or where to start troubleshooting this.

any ideas?