Growing up in the Bowling Green neighborhood of Sacramento, I was taught how to read and mark houses with the Halloween Candy Code. For kids with an early curfew these codes were invaluable. Once we tagged a house, our peers could use our marks to reap the best full-size chocolate bars while avoiding Chex mix and dried apricots.
Most marks were left in bright chalk at the bottom of the driveway.
I guess kind of like the marks hobos and tramps use to put on people's houses back in the day, like "lady gives food" or "mean dog behind fence" or "a cop lives here."