Two crazy thoughts... First, sometimes I'm startled to see my computer wake up from sleep by itself -- and then I realize I've bumped the mouse. Is there any input device or peripheral attached to the computer that you may be unknowingly jostling from time to time?
Second, I once had a wireless mouse that would occasionally seem to start moving around by itself. My theory was there was some kind of interference that was working on the same frequency or channel as the mouse. I stopped using that particular mouse in that particular location.