WHEN BACKING OUT OF MY GARAGE AT 2 MPH I STRUCK A TRASH RECEPTACLE THAT WAS LOCATED TOWARD THE PASSENGER SIDE OF THE VEHICLE. THE MERCEDES BENZ (MB) PARKTRONIC SENSOR DID NOT SOUND AN AUDIBLE ALARM UNTIL IT WAS TOO LATE TO REACT. FOLLOWING THE INCIDENT I TESTED THE AUDIBLE ALARM SENSOR FOUR TIMES AND DISCOVERED THAT THE ALARM DID NOT SOUND UNTIL THE VEHICLE WAS 3", 9", 9", AND 12" FROM THE TRASH RECEPTACLE. THE OWNER'S MANUAL STATES THAT THE REAR CORNER SENSOR ACTIVATES AT 32" - BUT IT DOES NOT STATE WHICH TYPE OF SENSOR ACTIVATES, I.E. VISUAL (FLASHING LIGHTS) OR AUDIBLE. I BROUGHT THE VEHICLE TO THE DEALERSHIP TO REPLACE A BROKEN TAIL LIGHT COVER AND TEST THE SYSTEM. I WAS INFORMED THAT THE SYSTEM WAS WORKING AS DESIGNED, AND THAT THE AUDIBLE SYSTEM DOES NOT SOUND UNTIL YOU ARE EXTREMELY CLOSE TO STRIKING AN OBJECT. THIS INFORMATION IS NOT CLEARLY IDENTIFIED IN THE OWNER'S MANUAL OTHER THAN TO STATE THAT THE REAR CORNER SENSOR ACTIVATES 32" FROM AN OBJECT. SINCE I RELY ON THE AUDIBLE SENSOR AND NOT THE FLASHING LIGHTS ON THE DASHBOARD, THE LACK OF CLARITY IS QUITE SERIOUS. HAD THERE BEEN A CHILD IN THE DRIVEWAY AND NOT A TRASH RECEPTACLE, I WOULD HAVE POTENTIALLY STRUCK THE CHILD CAUSING SERIOUS INJURY. AT NO TIME WAS I EVER INFORMED OF THE DIFFERENCE IN DISTANCES BETWEEN THE VISUAL AND AUDIBLE SYSTEMS. FURTHER, THE OWNER'S MANUAL DOES NOT DIFFERENTIATE BETWEEN VISUAL AND AUDIBLE DISTANCES WHEN DESCRIBING THE PARKTRONIC SENSORS. AFTER TESTING THE PARKTRONIC