@Marins an equally important question is where is the user logged in at.
If just a forum, blog site, etc you really have no way of forcing the user to hit log out
But if in a big company, or a State Government that user better log out.
In the State of Texas, Dept of Human resources it's State law to log out. If you don't you are subject to possible jail time and a hefty fine. You can verify this for yourself. The system deals with medical, thus very strict.