Automatic logout after 30 minutes inactivity

After 30 minutes of inactivity, Mac mini pops up a window "Are you sure you want to quit all applications and log out?", waits 30 seconds then quits all applications and reverts to login screen.

I checked all over System Preference but could not find any settings to effect this behavior. I even uncheck Require password _____ after sleep or screen saver begins, to no avail.