Some wireless desksets actually turn themselves off after a period of inactivity. If yours is on all the time the only thin I can suggest is removing the batteries. Have you downloaded the software for the mouse - it may have a power saving feature.
This may sound odd, but worth considering. Is your mouse on a solid desk/table? If not, whilst you are not using the pc, just walking near it could set it to start detecting. I had this problem a while ago when living in a flat near a road of thundering buses! Each time a bus went by, the detection light on the mouse turned on!