Dented Gold Coin

It is not uncommon for a gold coin to have a few dents. Usually, this happens when traders bite the coin, to make sure it's not fake. A coin with seemingly random dents is also a good place to hide a secret message in plain sight.
The Black Fist, a mysterious criminal organization has most of its operations split into cells. These cells know almost nothing about each other, so if one cell is compromised, it does not impact the other. Since communication within the organization is a necessity, it is important to do it as anonymously and subtle as possible.
One way The Black Fist does this is by placing secret messages in gold coins. The coin is dented in certain patterns, known only to a few.
Due to the relatively small amounts of dents, one can make in a coin without it looking suspiciously, the messages are fairly simple, usually only representing a few letters or numbers.
Since gold coins are fairly soft, no special equipment is needed to create the message. Simply by biting the coin in the correct pattern is sufficient.

"What does it say?"   "Give me a second ... W12"   "W12? What does that mean?"   "It's an address, or more precisely it's a sewer grate."   "A sewer grate?"   "They're all numbered. W means West, and 12 is the grate number."

— Conversation between a seasoned and a new agent of The Black Fist

The coins are usually given to street urchins, who are then asked to deliver the coin to someone or somewhere. Initially, this didn't work very well, as the temptation of holding a gold coin for a street urchin was too much to resist. This issue was however quickly rectified, once word about what happened when you crossed The Black Fist got out on the street.
Sometimes other methods are used, like hiding the coin at certain dead drops, or 'accidentally' dropping the coin at the right place and time, all depending on the nature and target of the message.
Once a message has been received, it doesn't take much more than a few bites into the coin, to destroy the message.