If it's something like that, chances are it's some sort of law enforcement or city official taking notes about your graffiti. The placement, size, height off the ground, etc. They take notes on how big you paint, and guess how tall you are. Essentially in some situations, they'll set up a grid on a map to see where you've hit, and where you MIGHT hit. Don't put anything past these fuckers. Unless you live in a crime-ridden inner city area, chances are the cops get bored, and since they're pubic servants, if the right person bitches to the right clerk at city hall, then guess what, you're under investigation.
Have fun! Welcome to graffiti!