The term 12 is derived from the radio code 10-12, which is often used by the police on their scanners. This code means that “visitors” are present in the area so discretion is advised, referring to possible suspects that the police have their eye on.
However, this slang term is used by non-cops to warn others that an officer or officers are in the area and approaching the scene of illegal activity. So, civilians take the meaning of the radio code and apply it to the police in the reverse way that it’s intended, which means the police are called 12 as a secret way to warn against their presence. Although the term originated as a warning, 12 has been somewhat adopted by popular culture particularly in the southern United States and around Atlanta, Ga. This is similar to one of the many lores behind people throwing pairs of sneakers on power lines. Although this act has different meanings around the world, one of these is that it acts as a marker that the police have been in the area as a warning for those who know of illegal activities happening.
This also acts as a marker for drug dens in order for them to communicate with their associates; however, there are many other various and non-illegal meanings behind throwing shoes on power lines. Another instance of similar police slang is referring to them as the “5-O.” This term actually comes from the television show Hawaii 5-O, and it is also used as a discreet warning that police are in the area.