Chuunins working at the mission desk and archives are watched by Anbu and some of them have to be sensors like Iruka. Imagine them having a unique name for each Anbu that watches them, because they can sense the same chakra, but not see their masks. And also having a special code like two taps on the shoulder or a number/word mentioned in a conversation means a specific Anbu is on duty this time, because even if they have the same rules stated by the Hokage, some react different to the same situation. Thanks to the secret communication they know when to not do/say specific things.
















