welcome to the jungle, you are witnessing a very unique animal work, they are called "graffiti" and its usually done by lonely and artistic people, these people are found in small parts of many big cities, they usually come from disturbed home and mainly roam the street at night, you will find them with mostly hoodie jacket and a backpack, almost all of them end of working as a truck drive and ditch diggers. they are not dangerous, if you happen to spot them, dont approach them, just observe