Her account is still active or was at 10.13pm last night. Don't mean it's her behind the keyboard and I don't know what actually triggers an active session,
In general with web programming it's a connection between browser and server and would time out if the browser didn't get another request. In the case of the forum, I would suspect when someone logs in it associates the generated session Iad with that user.
If you're seeing session active then that means they are doing something on the website that prompts the client (browser) to talk to the server. If someone were to die while on not606 their session would disconnect when the server timedout the session. It wouldn't show them as active (even if the PC stayed on and the browser window open for some reason).... Unless xenforums are specifically programmed to keep pinging the server while browser active (which would be inefficient, so unlikely)
Membership has it's privileges.