canary in a 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 a 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 a coal mine to see its effects on the human mind.
look like the cat that ate the canary
1. To appear smug and/or overly pleased with oneself to others, especially over something one has done, received, or accomplished. After her promotion, Janet spent the rest of the day looking like the cat that ate the canary.
2. To have the appearance of guilt but attempt to hide behind an air of smugness or feigned nonchalance. When questioned on the use of campaign funds for his own personal pleasure, the senator looked like the cat that ate the canary.
miner's canary
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 was present, thereby alerting miners to the danger. Wildlife in disaster movies always acts like the miner's canary, fleeing the scene when catastrophe is imminent. John was used as a miner's canary to see the test drug's effects on the human mind.
sing like a canary
To inform against someone to the police or other authority about his or her criminal or illicit behavior. I heard Joey Malone has been singing like a canary in the hopes of getting his sentence reduced. Let's make sure he's sleeping with the fishes before he gets the chance!