The light under the orange button is for the aft fog light which is available in some countries (not in US).
If you notice, your light switch has *two* detents as you pull out the knob when turning on your fog lights...actually the first detent turns on the front fog lights, and the second detent turns on the aft fog light (in those countries that it is available).
If you look at your tail light assemblies, you will notice that (in the US) there is one *blank* light position on each of the tail assemblies. Well, that position will contain (in non-US delivered versions) a rear fog light on the drivers side only..left for LHD, right for RHD cars. It is an extra bright bulb to help people from running into you from the rear in dense fog.
The orange button light will illuminate, if you have the aft fog light on.