Don't know about you, but I have this power-saving "do stuff half-awake without remembering any of it" feature. I've turned on the computer quite a few times like that.
Well I was certainly still in bed when it happened so no, it can't be the case. :-)
What comes to mind here is in the bios there is an option to reboot if power is lost. Occasionally this can boot your machine up from a power off state.
So you may have had a slight dip in power or a power cut for a few seconds, when the power has returned to normality the PC has booted itself up.
+1. I use this feature on my customer's machines, so they can turn on the PC with the main power switch instead of turning on the PC separately after the main switch is on. Saves a little time and keeps them from opening the PC cabinet door and letting in dust and dirt.
Edit: it's called something like "Power on after AC loss", and then you choose yes or no.
This sounds plausible. I'll check later if I have this checked.
Also, it certainly wasn't sleeping, it was shut down as much as it could be. Only hammer could shut it more. :-)