The Blind Spot Monitoring System on the Mazda CX9 is a RADAR sensor system which sits BEHIND the plastic bumper, so it is completely invisible from the outside. The system detects and warns for Dynamic Objects which are moving in the "Blind spot zone". The first level of warning is simply a yellow Icon lighting in the side mirror. If, however, you use your signal and a moving object is in the blind spot in the direction you signal, a second "audible" alarm will warn you. I believe the Mazda website has further information about the BSM.