Yes. It’s off trail. It is in the Mill Creek drainage above where the Vivian Creek Trail crosses Mill Creek at the beginning of the trail. The first graffiti appears about a quarter mile up the wash. Then it appears periodically all the way to the Mill Creek Jump Off. It’s difficult to tell, but the second picture above is of a rock in the center of the Jump Off at just below the beginning of steepest section of the headwall.
The puzzle of it all is (maybe I’m stereotyping here) my experience is that people tag areas that are easy access and highly visible (see Big Falls). High visible at least–I am often extremely impressed by how some access pretty crazy places in the city. This idiot had to work very hard for something that few will see. I think quite a few people do the water fall hike, but above the falls to the top of the Jump Off — not too many.