Wiktionary
n. A burglar alarm that makes no noise that is audible to the trespasser.
Wikipedia
A silent alarm is a burglar alarm that makes no noise that is audible to the trespasser. The alarm makes an audible noise elsewhere and notifies the police. A silent alarm may also be a panic button alarm.
The term still alarm is used for a fire alarm transmitted silently, usually by telephone, rather than by sounding the conventional signal or bell apparatus. Still alarms are commonly used to alert fire and medical personnel as to what type of emergency they are responding to.
Category:Fire detection and alarm
Usage examples of "silent alarm".
He was in the process of burglarizing a house in Troy when the police arrived, summoned by the silent alarm.
They found where the telephone lines came into the home, and cut them, to forestall both a silent alarm to a security company, or a call to the police by a frightened victim.
Even now a silent alarm might be at work, summoning bodyguards or security from a nearby suite or downstairs.
Yet his tone was so changed, so strong, that it sounded a deep silent alarm in me.