You can see the last point in action on a number of places. I can think of two sites in particular that pop up "You will be signed out in XX [decreasing number

You can see the last point in action on a number of places. I can think of two sites in particular that pop up "You will be signed out in XX [decreasing number] seconds unless you click 'Continue'." messages. I can come back to the site's tab three hours later, and the countdown just started!