Other forms: red flags
A red flag is either a literal warning of some danger, like the signal flag used by a sinking ship, or a figurative warning, like the red flag a candidate's angry outburst sends to the voters about his temperament.
Red flags have signaled dangerous situations for centuries, from seventeenth century military flags marking the start of a battle to a lifeguard station's red flag, warning beachgoers about dangerous water conditions. Even some spam filters refer to their junk email alerts as red flags. Figuratively, behavior is often a red flag: "He ate all my food and left the toilet seat up — it raised a red flag about choosing him as a roommate."