I have a strange issue with my MacBook Pro (6,2 OS X 10.6.8). When I leave the laptop for some time, let's say 30 minutes, I find only the log in screen.
I think it did not reboot. It just logged me off. Luckily some programs reject to be closed (that's annoying too, but different story) an I find a dialog that tells me application X prevented logout.
I am aware of the system setting to automatically log the user off. It is not active. I checked the GUI a hundred times.