Watermarking a city consist in putting signs readable (series of symbols on sidewalks and walls to indicate something.) only by happy-few :) like hobo-language, warchalking, graffiti, rave codes... It seems that warchalking was inspired by the practice of hobos during the Great Depression to use chalk marks to indicate which homes were friendly : "a code through which they gave information and warnings to their fellow Knights of the Road"