Random unread emails

One of my clients has a random issue with her emails in her inbox.  She has emails that were read that show up as unread.  The emails have been read because she either forwarded or replied to them and she has a reading pane that shows emails as read in 2 seconds.
Just to be sure that she had in fact read the emails, I marked all the emails in her inbox as read and the next day, when she opened Outlook, she had random emails from previous days and weeks that appeared as unread, more than 20 emails were affected.
She is using Outlook 2010 with Exchange 2013
Now that I have proven that the emails were read and then marked as unread, I have no clue what the next step would be to fix this issue other than what I have done already which was to do a reset views, reset folders, scan the pst file, repair/reinstall Outlook.
Furthermore, there is no link between the emails that mark themselves unread, they are not from the same time of the day, they are not all from external addresses, some are from the same company, they do not have similar subjects or topics.
Any suggestion would be greatly appreciated
Thanks
Jeannie