When you leave the site after 5 minutes you will be logged out automatically, although you can manually log out by going to Your Details > Logout if you wish to.
If you return without deleting cookies, the system logs you back in automatically (That's not to say, you were still logged in from before (otherwise we would have thousands logged in permanently!)).
If you delete your cookies and return 5 minutes after your last session you will see you have been automatically logged out.
We do hope this helps, if not please do get back to us so we may work together to resolve this issue for you.