Schneier: How do you tell the system on the other end of the line that you re no longer there? He says many office computers are left logged in when people go to lunch, or when they go home for the night. Some systems have experimented with a token: a USB authentication token that has to be plugged in for the computer to operate, or an RFID token that logs people out automatically when the token moves more than a certain distance from the computer. Choosing an unauthentication method depends a lot on how the system is used as well as the threat model.
Source: https://threatpost.com/difficulty-un-authentication-092809/72267/