I will answer for Black/White 2 now and update my answer later if/when I find any more.
For the medal in BW2 you need to read the back of the sign at the north of Route 2.
Regarding other versions, I definitely don't remember any graffiti in any of the generation 1 or 2 games. I'm sure there is some in gen 4/5, not sure about gen 3. Anyone else feel free to contribute to this answer :D