I have a suspicion that this has something to do with browser settings. My browser (Firefox) is set to open links in a new window. When I use the forum's Google search function and select a result the browser opens a new window with me showing as still logged-in. If I then select a link in the new window (ie, thread) it'll open yet another window showing me as logged-out. If I then return to the original search window and click the back button I'm shown as still logged-in.
Alan