Well I just took a look. Stayed on that rock in different positions, sat down so the camera could look around more, turned the brightness up and down on my tv,all sorts of things. All I come come up with is that's it's the reflection on parts of the rocks below. I could not recreate the image really. So my best bet is as I said above. A really lucky camera angle just happened to line up in the question mark shape unfortunately. If you find anything more please let me know;)