fire alarm vs red flag

fire alarm

noun
  • The sound or other warning made by a fire alarm. 

  • An electromechanical or electronic bell, klaxon, chime, horn, speaker, strobe light or other device which warns people in a building of a possible fire or other condition requiring evacuation. 

  • An emergency response to a fire by the fire department with manpower and equipment. 

red flag

noun
  • A cue, warning, or alert; a sign or signal that something is wrong. 

  • A signal that the race must be stopped immediately (and possibly restarted). 

  • red rag 

  • A common symbol, usually in the form of a banner, for leftist revolutionary ideologies, especially socialism. 

  • In the United Kingdom and some other countries, a flag formerly used by a railway guard to warn the driver not to start the train. 

How often have the words fire alarm and red flag occurred in a corpus of books? (source: Google Ngram Viewer )