There have been a couple of other reports of this type of thing. Nothing has really been determined, though.

Is it possible that you went out to the car at some point and switched the ignition on and quickly back off? That would make the empeg start up but not see the switch off, which could cause this if, for some reason, you didn't hear it as you were leaving (remember that it takes a while for the empeg to boot up and it's possible that it was on a quiet song right then).