A security device used to alert outsiders of an emergency situation, usually to call the police in the event of an armed robbery. They appear at banks, convenience stores and any place that accepts many cash purchases. The alarms take the form of small buttons or switches placed inconspicuously in reach of a worker’s hands or feet, so that the alarm can be triggered quickly without attracting attention from a potential robber. Instead of activating loud sirens and flashing lights, a signal is sent to a local police station. This isn’t like a fire alarm that will alert everyone inside the building to evacuate – the goal here is to only notify outsiders, and fast.

Once activated, the silent alarm stays on until police respond. They will call to ask if everything is OK, all the time assuming that something is wrong. Since robbers might be holding up employees, they might not be able to respond truthfully over the phone. So police will always respond, even when someone accidentally triggers the silent alarm and later denies any trouble.

Recently, taxis have adopted similar technology. The next time you see a taxi drive down the street, look carefully at the light on top. There will usually be a main light reading “in service” or “on call” that turns on depending on whether the driver is on duty. On the sides of the main light, there is sometimes a “call 911” indicator. The driver can activate this light with a hidden button or switch if they feel threatened or are being robbed. While it can deter robbery, this system is not as effective as the bank systems, since a potential robber could look at reflections in nearby windows to see whether the security light is on. Also, the driver relies on bystanders or other motorists to call the police instead of having a signal sent directly to the local PD.