Earlier today, I discovered this https://dl.dropboxusercontent.com/u/117697726/image.jpg and first of all thought that it was some sort of sensor. It has like a see-through matte surface, up in the hole. But I've never heard that the vision plus v3 should have a built-in sensor of any kind. Any of you guys know what it is?

RC link status indicator.

That is the button you press to link the Transmitter to the Phantom.
Page 19 of the latest manual shows the procedure.

LOL sensor, someone didnt read the manual

Wow!  I was afraid for a minute you were  going to give away the real meaning.