We implement Trac with HTTP/S Basic Authentication. When you log into Trac, your browser caches your auth credentials until you close it. There is no cross-browser way to make an HTTP Basic Authentication session log out by clicking a link other than forcing a browser close, which most users would find undesirable. We are evaluating ways to better manage this as we move closer to a single sign-on mechanism, but in the meantime, it means the logout link does not take any action, and you should close your browser after a Trac session if you're on a shared computer. We're sorry about any problems this causes.