When you are using ESP (Edge Security Pack) for a Virtual Service load balancing Exchange, the login sessions are controlled by the LoadMaster. This gives the LoadMaster the ability to control the amount of time a user can be idle before the LoadMaster will end the session. This feature is set when you specify the Use for Session Timeout drop-down to Idle Time. This is found inside the domain settings in your Client Side SSO Domain. See below for an example.
The LoadMaster tracks idle time based on any packets that are transmitted across a specific TCP stream. For normal applications, when you close a tab in the browser - this closes the connection for that session.
For Microsoft Exchange however, if you close the tab in your browser connected to Exchange, it will continue sending keepalive packets to the LoadMaster, keeping the session open for an extended amount of time. When you try to log out of Exchange 2013 through the LoadMaster it will warn you to close all open browser windows, so the keepalives will stop.
In order for the Idle Time to begin counting, you must close out the browser completely to ensure Exchange is not keeping the session alive. Simply closing the tab is not enough.