They just don't explain products anymore. http://www.instructables.com/id/Silent-Optical-Mouse-No-red-shining-light/ gets into the nanometers of the wavelengths used.
Could be that simple.
In the past the optical mouse have a red infrared light on the bottom. This I can see when the mouse is functional. Now I don't know why the new design of optical mouse does not have visible light. Is it an improvement or cost cutting design? I would have to test with a computer to see if it has run out of battery or became faulty. I asked the Logitech vendor but their cs is useless... they didn't give me an explanation.