Just discovered this as I used the Google ID signin process to get into SO. If I sign into SO using my Google ID, I am instantly logged into every other place where my Google ID is used.
I can view my mail, my google docs, and I show up as logged in on Google+ (doesn't matter because no one uses that). But it leads me to this question - Why does logging in on StackOverflow with my Google ID automatically log me into every other Google service? I didn't want to log in to any of the other Google stuff, only StackOverflow. If I want to stay offline in Google+ and logout, I'm logged out of Stackoverflow. Is there no way to log me in here with my Google ID without logging me in on every other application?