I'm checking on Management Console, and for my document, the last connection of the assigned user is at 9:32 today, but this user logs on this document each 15min. The last connection of the user were at 11:17.
It look like the Document CAL is not always used by the user.
The question you may ask yourself: is the CAL usage line updated for every single access, or just at the beginning of a session. If your user accesses the document every 15 minutes, the session will not be closed automatically (if the QMC Session Timeout value is still set at 30 minutes).
How to test? Ask your user to explicitly close a session every 15 minutes by clicking the close button in the top right corner.
Anyway, if the QMC reports that a CAL is in use for that user and that particular document, then that information is correct. AFAIK there is no technique to open a QlikView document in the AccessPoint without using a license (or we would already know about that )