Seems to still be happening. Today I found that it had happened three times. Once when I checked at 8am, then 12pm and finally at 6pm. Each time the screen was basically the login screen / lockscreen. One thing I have been keeping track of is the system uptime. After I rebooted this morning at 8am, my systeminfo.exe report shows the system as being active since 8:04am. Even after the next two events it's still showing 8:04am. To me this means that Windows is not crashing and rebooting. And even if it did, I have it set to auto login my username so I would expect to be fully into Media Centre.
I run Media Browser and Plex as plugins within MC but I've set those apps so that they don't sleep or logoff the PC. The event viewer logs show some info but I'm not 100% sure if it correlates to what's happening. For example, after watching a show via Media Browser at lunch time, there are logs in the Security section showing Logon and Logoff for an account that I have no idea about (DWM-2). After that, nothing until I login at 5:50pm with my normal account. I can post a pic if it will help.