According to Studio 360, an unknown artist has been leaving messages in the streets of Philadelphia. I have been seeing these myself in New York as well for many years. I took an image of this “Toynbee Tile” a few years ago, and there are a few more in the Pavement series. A documentary film speculated that the tiler remained unseen by dropping the tiles from a car. Somebody made T-Shirts with Toynbee Tiles.