A customer was seeing an issue where they would open an Outlook Email Message from NetDocuments and close it, then when they clicked the Logout link, it would give them a message telling them that the email was open even though they had already closed it:
EMAIL NAME is open, so it cannot be checked in. Either close the document and click OK to check it in, or click Cancel to leave it checked out.
It turned out that the user was using an old version of the Mimecast add-in. Once they updated it, they no longer were seeing this behavior when logging out.
I have also seen this happen with an Outlook add-in called ContactMonkey. Once we disabled this add-in the user no longer saw this error.