Well, my iPhone 4 did this around the 18 month mark and blow me if my iPhone 5 isn't doing it too! Same place! Just right of the volume down button. The iPhone 4 'splodge' started exactly like this and grew three or four times in size over the following six months. Is there any particular reason why they go yellow in that part of the screen?? The strange thing is that it doesn't show on a screen dump (below)! Why's that?