After a posting session from a publicly owned terminal, how to log out ?
First, click "log out" in the "Site Switcher:
The page will helpfully remind you:
If you're on a shared computer, remember to log out of your Open ID provider (Facebook, Google, Stack Exchange, etc.) as well.
Oddly, there is no link to log out of SE Open Id on that page. So here you have it.
(If that link gives you a "page not found" page, then you're already logged out)
Or, you could wipe out all local data from the browser: cookies, history, etc. This is what I would probably do.