I have a regular computer mouse and a clear layer of plexiglass on my computer desk (normally I use a mouse pad). If I place the mouse in certain spots, I've noticed the pointer may move on its on a bit.
Being interested in why this happens due to thinking the mice shouldn't work on clear tops, I've tried different mice and find that the sensitivity varies. I understand how they work, an LED shines onto a surface and a camera on the bottom of the IC detects objects such as lines changing direction.
I believe the plastic over the camera lens is to magnify, but not certain. The LED is reflected at a specific angle that is directed to the camera.
As for the sensitivity where it moves on its own, but not consistent at different locations, why would this happen? The surface isn't changing, and, if it's a matter of light reflection, it should should always the same reflection angles since the mouse isn't moving.