I don't think it really matters whether it's real or a hoax. If you live in areas plagued by gangs, it would be worth it to be on the look out for that type of activity in your area.
It's certainly plausible. A lot of gangs have a pretty well run criminal enterprise going, and are pretty cunning to boot.
While I've never thought of that particular activity, I've never been a fan of the more obvious ways of tagging your own vehicle by putting on gun related bumper stickers like NRA stickers. I keep my vehicles clean and devoid of such advertising.