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 ?
That's raising a red flag among some elected officials and business leaders, who wonder why so many government dollars are going toward a development that will in all likelihood bring no new major tenants to downtown St.
For Berns, as his other writings make clear, it extends to censorship of "immoral" materials (which is certainly a red flag).
A yellow or white flag is attached to one end of the shaft and a red flag to the other.