Ok here's one for people to look for, my iPhone 6S Plus has it; as does my wife's. I am not going to return it as I believe its a design issue rather than a fault. I checked a few demo units in the Apple Store I was in, and they all had it too. Have a look at your iPhone 6s Plus screen carefully, about 2/3 of an inch up from the bottom, on the left hand side, there appears to be a pressure point, fairly faint, which causes a slight light spot on white or bright screens. I don't have a camera with me I will try and post it later. Not trying to start a "gate" or anything just an observation.
Edit: Added a photo for those interested. Sorry for the abysmal quality.