Unless you've tested OP's fiancee's system yourself and found that to be the case, I wouldn't be so sure.

I had a Logitech G500 with awful coil whine and the opposite problem: it would stop whining when moving and start when idle. I suspect it had to do with the power saving mode that lots of mice have, where the laser power supply ramps down to dim the laser illumination after a period of no detected motion.