I came across this image recently, that claims the existence of a code known by criminals. The post does not mention the area where this code is supposedly used, but it can be inferred it pertains to unorganized burglars.
It's easy to think that this code would go out of vogue with the Internet, but even so there's something about the idea I find too romantic to be true. Here's a similar code used in the episode of Mad Men titled The Hobo Code:
There are many more images like this found on the web. I mentioned I find the idea too romantic to be true, here's why:
The code is supposed to be known by a certain class (caste?) of people, not shared between friends. "Doing a solid" to unknown hobos or criminals, if such a person oneself, seems awfully well-intended, seeing that there's nothing to be gained by it.
The code must be secret to have the intended effects. It is very easy to leak.
Once leaked, it could be gamed by the police or anyone else. Even "legitimate" code consumers can set up traps for their fellow hobos or brothers-in-crime. Thus, people would learn not to rely on it that much.
I will allow that it's possible for such a code to have existed at some point, before being written off as a Bad IdeaTM.
So, the question: Has there ever been any evidence of symbols marking property, that is known to describe the property's owners or content, intended to be used by criminals?