Once you have logged on to E3 you can log off again using the 'Logout' link, which can be seen by holding your mouse over the 'Home' link in the navigation bar near the top left of the screen, or within the navigation bar at the left of most screens.

If you neither log off in this way, nor close all of your browser windows, then anyone who has access to your PC may use E3 without having to enter your password, up until the point that your E3 session expires (this will be 30 minutes after you last accessed an E3 page).