Methods, systems, and apparatus, including computer programs encoded on a storage device, for using a drone to test a sensor. In one aspect, the method includes actions of detecting a message (i) broadcast by the drone and (ii) indicating that the drone is going to administer a test of a sensor, determining, by the monitoring system and based on the message and (i) sensor data generated by the sensor in response to the administration of the test, by the drone, within a predetermined period of time of the message or (ii) a lack of sensor data generated by the sensor in response to the administration of the test, by the drone, within a predetermined period of time of the message, whether the sensor is functioning properly, and in response to a determination that the sensor is not functioning properly, storing data indicating that the sensor is not functioning properly.