While experimenting with placing my FasTrak toll transponder, I noticed that on the driver's side of the rear view mirror, on the forward-facing side of the mirror, is a small recess just about big enough to put the tip of your small finger into (*not* that I am recommending that you do so! ). At the bottom of this well, there appear to be two distinct optical elements, which seem to be red or green at various times. Here are some rather dreadful photos (apologies for the poor focus, but it was very difficult to manipulate a phone cam in that spot, and I don't have a macro lens for it, either). What is this gadget? What purpose does it serve?