On the side of the Dome over St Peter's--the side opposite the mosaics--is a stairway to the roof. That stairway, when I was there, was sheetrocked and unpainted with 100's of graffiti scriblings in all manner of languages. It was quite a contrast to the mosaics inches or so away. But it was also very human.
So you are speaking of actual present-day graffiti? I thought you were referring to the verses from Matthew inside of the dome.