This is a photographs of upper right corder of dash board. My model is Prius GenII S (10th Anniversory S JDM).
Solar sensor which provides an input to the air conditioning amplifier to help the AC amplifier decide cabin fan speed and AC compressor speed, when A/C AUTO is used.
And if you get auto headlights like we do here, the solar sensor turns your headlights on/off if the switch is in the auto position.