3 ideas
At Heron's Park, a Kent military hospital in the Blitz, the postman Joseph Higgins dies on the operating table during routine surgery for a fractured leg. The murderer had repainted a carbon dioxide cylinder in the black-and-white livery of oxygen. The anaesthetist, reading the colours, gave the patient CO2 while believing he was giving oxygen. The killing ran entirely through a trusted colour code, with no one touching the patient with visible malice.
A visual standard exists so experts can act fast without checking. Anyone who can alter the signal therefore controls the expert's action. The more reliable a convention has been, the less anyone verifies it, so tampering with the label is more dangerous than tampering with the contents.
In a closed group, nearly everyone is hiding something small and unrelated, such as an affair, a professional lapse or a private grief. Each of them lies to protect it. Those innocent lies generate as much suspicious behaviour as the guilty party's, so the investigator's work is to sort the lies by what they protect rather than to hunt for liars.