If your vehicle rapidly closes in on the vehicle ahead because the vehicle applies the brakes suddenly while you are traveling in headway control, the warning sound activates and the brake warning is indicated in the display. Always verify the safety of the surrounding area and depress the brake pedal while keeping a safer distance from the vehicle ahead.
Additionally, always keep a safer distance from the vehicles behind you.

Mazda Radar Cruise Control (MRCC) Display Indication
Setting the SystemMazda 6 Owners Manual: TCS/DSC Indicator Light
This indicator light stays on for a few seconds when the ignition is switched
ON. If the TCS or DSC is operating, the indicator light flashes.
If the light stays on, the TCS, DSC or the brake assist system may have a malfunction
and they may not operate correctly. Take your vehicle to an Auth ...
Mazda 6 Owners Manual: Add-On Non-Genuine Parts and Accessories
Non-genuine parts and accessories for Mazda vehicles can be found in stores.
These may fit your vehicle, but they are not approved by Mazda for use with Mazda
vehicles. When you install non-genuine parts or accessories, they could affect your
vehicle's performance or safety systems; the Mazda w ...