Like a bar-code, but made of concoctions of smelly chemicals.
So, we've got litmus paper, that changes color based on pH. However, there's other types of indicators, that change color based on gas present in the air. For example, lead acetate paper strips indicate H2S presence in the air.
So, imagine a piece of multi-stripped paper, that changes color based on the presence of a variety of compounds in the air, that change the pattern, like a bar-code or a QR code, based on the presence of a diversity of compounds in the air.
These papers would be a way to visualize the smell-codes, but the smell-codes themselves would be just that abstract concept, that information can be encoded and transmitted in smells, and a dog may not even need to have such paper to read messages coded in smellcode.
These codes may have many applications, including scientific communication of smells. Also, a specific smell-encoding could be defined, to write and read such-coded messages, that may be universally usable, say, even after the message encryption.