silent alarm

noun

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.
any visual rather than audible alarm.

And then, some silent alarm clicked into place in Sabo's mind, tightening his stomach, screaming to be heard.

“The Silent Alarm,” she murmured as she hastily drew from her pocket two dark cylinders.

By the aid of the Silent Alarm she had received a message from her.

Until now the Silent Alarm had been little more than a plaything.

Maybe that's why I didn't show up any better, here the other night, when Vee rings in this silent alarm on me.

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