canary in a/the coal mine

Something or someone who, due to sensitivity to his, her, or its surroundings, acts as an indicator and early warning of possible adverse conditions or danger. Refers to the former practice of taking caged canaries into coal mines. The birds would die if methane gas became present and thereby alert miners to the danger. Wildlife in disaster movies assumes the role of the canary in the coal mine, fleeing the scene when catastrophe is imminent. Unaware that he had been given the test drug, John was used as a canary in the coal mine to see its effects on the human mind. This guy's usually my canary in a coal mine. If he's reporting an issue with the system, it's worth looking into.