These are the hobo codes. Hobos have used these codes to communicate with each other about their surrounding. You don’t want a fellow hobo going into a hostile situation, so you give him a warning. You can also let him know everything about a town before he walks there. If you live by a train yard you can probably find a few of these around still today.
But with the rise of us online hobos, we have made a new set of internet codes. They are top secret and posted below.