The auto-logout option appears at the bottom of the "Choose Staffing Assignments" dialog that appears when you log into the webclient for staffing.
After the login duration is reached, you'll be alerted and given a chance to extend and/or modify the timeout. If you don't respond within a minute (because you are actually gone for the day or maybe you've wandered away while helping a patron), the webclient will log you out.
What if you don't enable auto-logout and accidentally leave yourself signed in?
If you realize you've left yourself logged into the webclient on a computer you cannot physically access, you can opt yourself out of your queues. Sign into the webclient from another location and opt yourself out using the regular queue selection box. This will opt you out of your queues from all locations no matter what software client you are using. Phew! Then, logout as usual. You'll be able to opt yourself back in when it's time for your next shift.
An administrator can also opt users out of queues. In the users management page (US, CA, EU, SG), select the user in question and uncheck the boxes next to the queue assignments to temporarily unassign users as operators on a queue.
What if you use another software client to staff your LibraryH3lp service?
If you staff your service through another XMPP (Jabber) client such as Pidgin or Adium, no worries. Locally installed desktop clients usually have great support for automatic setting of Away status based on keyboard/mouse idle time. So with these clients, you can prevent yourself from falsely holding the service online easily.
But if you happen to not have auto-away enabled, or if the trigger is a long way off, you can still use the admin dashboard or webclient to opt out of your queues. Just follow the steps in the section right above this one.
One related note. You can also have automated replies based on a lengthy wait time for your guests. This helps cover things not only if your service has been left online accidentally, but also in case your operators can't respond as quickly as usual for any other reason.
 The auto-logout feature is not yet available on our regional Canadian service.