I purchased a used 2015 mazda6 in 2020. on the backup system/camera, should there be a warning when getting close to an object directly behind the car? I only hear the beeping when something on the side is detected. when backing up in a parking lot, if a car is approaching down the aisle I'll be warned. but a wall or garbage car behind the car? radio silence. is this normal or yet something else failing in a system?