silent alarm

noun

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.

Example Sentences

Twice the operation was interrupted by police officers who came to Sinay’s office when she accidentally set off a silent alarm, but for the most part the copying proceeded without mishap.

From Los Angeles Times

Police contacted Ms Leonard after a priority 999 call from a silent alarm system in her home had been logged.

From BBC

China’s nosedive in childbearing is a silent alarm.

From Washington Post

An estimated 43 percent of public schools have a “panic button” or silent alarm that connect directly with the police in case of emergency, up from 29 percent five years ago.

From New York Times

The teller walked away while another employee activated a silent alarm, and the man left without any money.

From Washington Post