It doesn't actually cost all that much to make some posters and buy the keys to do this sort of thing.

(Not that I'm saying anyone should do this).

(I'm not sure if people hack digital public ad space ¯⁠\⁠_⁠(⁠ツ⁠)⁠_⁠/⁠¯ too.)

Not sure if the keys have changed by now. It used to be more a thing... 'subvertising', guerilla marketing and all that. Never really gone away, but perhaps having some sort of revival? Seeing a bit more of it.