I just checked and it's not the sensor for diming the dash lights. As Nate says, that sensor is on the main instrument cluster screen. The more I look at this the more it looks like just a little plastic cap that BMW decided to put on the dash just to make people wonder what it is. And if so, it is certainly working!
Seriously though, it must have a function. If I had a BMW dealer close by I'd stop and ask them. If It is indeed an IR receiver (and it does look like it could be), I can't imagine what for.
2014 BMW i3 with Range Extender Engine, Capparis White/Frozen Black, 20" wheels,, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, TurboCord EVSE