a 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!
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
a red flagmainly 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.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012