Maybe it's a simple matter of having "power on mouse" in the bios.
If the mouse is on a shiny surface it will detect light changes and think it's being moved.
I had this with my HTPC, with the mouse on a glass table and mouse power on enabled.
Disabling it solved it.
It's worth to check it out.
edit: also check in device manager if "allow this device to wake up...." is checked for the mouse.