This is another instance of something caused by flash photography. Remember, spirits are ok, its Flash photography that's truely evil.
If you notice the white area on the "STOP" sign. That is reflective white paint, with reflective particles inside the paint. The white areas on the wall are the flash reflecting off the paint and creating the pattern. If you notice it even is taking on the vague shape of the hexagon.
I personally think the there is a sign in the picture, relating to flash photography. (STOP)