It has always happened, and on both of my computers - and each one is set up so that system standby is on "never". On both of them, if I don't use the mouse for more than a few minutes the screen goes black and when I touch the mouse, it comes back up, but I have to pick a user first before returning to the screen I was on. It must be something to do with how I have the users set up that it is doing that.