alarm vs keep an eye open

alarm

verb
  • To rouse to vigilance and action; to put on the alert. 

  • To give (someone) notice of approaching danger 

  • To surprise with apprehension of danger; to fill with anxiety in regard to threatening evil; to excite with sudden fear. 

  • To call to arms for defense 

  • To keep in excitement; to disturb. 

noun
  • An instance of an alarm ringing, beeping or clanging, to give a noise signal at a certain time. 

  • A mechanical device for awaking people, or rousing their attention. 

  • A sudden attack; disturbance. 

  • Any sound or information intended to give notice of approaching danger; a warning sound to arouse attention; a warning of danger. 

  • A summons to arms, as on the approach of an enemy. 

  • Sudden surprise with fear or terror excited by apprehension of danger; in the military use, commonly, sudden apprehension of being attacked by surprise. 

keep an eye open

verb
  • To remain vigilant for someone or something. 

  • To remain vigilant for a possibly dangerous situation. 

How often have the words alarm and keep an eye open occurred in a corpus of books? (source: Google Ngram Viewer )