What must the duct smoke detectors in required monitored facilities report?

Duct smoke detectors in monitored facilities are specifically designed to enhance safety by detecting smoke within the ductwork of HVAC systems. When these detectors are activated, they serve a critical function by reporting a supervisory signal. This supervisory signal indicates that smoke has been detected in the ductwork, prompting a response from the building's fire alarm system.

The purpose of the supervisory signal is to alert monitoring personnel or systems without necessarily triggering a full fire alarm response immediately. This is essential since duct smoke detectors are typically installed to prevent smoke from spreading through the ventilation system, potentially leading to more severe hazards.

In this context, the other options do not accurately reflect the role of duct smoke detectors. A fire alarm would indicate that a fire has been detected and would be a more severe alert. An emergency signal usually pertains to situations requiring immediate action, and a warning signal could imply a broader range of alerts not specific to smoke detection within the duct. Hence, the supervisory signal is the correct indication that duct smoke detectors should relay when sensing smoke.