Most mobos can be turned on by various activities - kbd , network, modem etc. These functions wouldn't work unless the various peripherals had at least some power to them - so they are on the standby power line which is on all the time unless you disconnect the power from the wall. If it is a cordless mouse, you should either switch it off or put it in its charging stand when not in use - voila, no more lights.
.bh.
:sun: