That's called Bas Relief. It's a well-known stone carving technique, but nobody's seen anything like that before. It's new to rock art and, perhaps, is only half made.
It could it be geological, a lense of harder material within the block that has stood proud when the stone has been dressed. These things are not particularly rare in sedimentary rocks, calcareous/ferruginous nodules and such like. In the drilling game we simply call them stringers.