the.com/canary in the coal mine

means A person, event, or small sign that serves as an early indicator of impending danger or trouble.

from From an actual mining practice: caged canaries were carried into coal mines because the birds are more sensitive to toxic gases like carbon monoxide than humans are. If the canary stopped singing or died, miners knew to get out before the air killed them too. The literal practice faded with the invention of electronic gas detectors in the mid-1900s, but the phrase survived and went fully metaphorical, now used for anything — an economic indicator, a melting glacier, a single struggling business — that signals a larger, less visible danger approaching.