raise a red flag

raise a red flag

To offer a sign or signal indicating potential, incipient, or imminent danger or trouble. Didn't the fact that your accountant used to be a drug dealer raise a red flag or two when you started doing business with him? 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, raise, red
Farlex Dictionary of Idioms.
See also:
  • a red flag
  • red flag
  • on red alert
  • a/the sword of Damocles
  • Damocles' sword
  • in a blaze of glory
  • two-by-four
  • tear out
  • tear out of
  • tear out of (something or some place)