It's never happened for me between browser sessions, and never before I log in for the first time each session. My cookies are set to expire at the end of a session.

This happens to me too, but I can't really tell if it is cookies or bisquits or liverloaf becuase it appears totally random.

I visited yesterday and was logged off, visited earlier today and was logged off, visited a few minutes ago again and was suddenly logged in. Same happened more constantly a couple of weeks ago.