a red flag

red flag

A sign or signal indicating potential, incipient, or imminent danger or trouble. There are a couple of red flags you need to look out for to make sure your company stays in the black. Well, it definitely raised a red flag when he got so angry at me over such a minor thing, but I never thought he could be so unreasonable to live with!
See also: flag, red

a red flag

mainly AMERICAN
You call something that gives a warning of a bad or dangerous situation or event a a red flag. These are devices that are necessary components of nuclear weapons, and clearly that has raised a red flag in the minds of a lot of people. Cholesterol was the red flag that alerted millions of Americans to the fact that diet really does matter. Compare with a red flag before a bull.
See also: flag, red
References in periodicals archive ?
Barnett said quarterback Mike Moschetti's game-week comment to a Denver television crew should have raised a red flag.