the.com/canary in a coal mine

means An early-warning sign — a person, place, or small event whose trouble signals bigger danger coming for everyone else.

from From actual coal-mining practice: miners carried caged canaries underground because the birds are more sensitive to odorless gases like carbon monoxide and would weaken or die before gas levels became lethal to humans, giving workers time to evacuate. The practice dates to the late 1800s/early 1900s in British mining and lasted in some places well into the 20th century before electronic gas detectors replaced it. The phrase jumped into general language as a metaphor for any small, sensitive indicator that signals a larger hidden threat.