Meaning of silent alarm

si'lent alarm'

Pronunciation: [key]
  1. an alarm that alerts security personnel or the police without the knowledge of the intruder or criminal whose presence triggers it: The silent alarm set off a flashing light at the police station.
  2. any visual rather than audible alarm.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.