How did an emergency vehicle sensor in a BMW work?

My brother had a 1989 BMW 735i and we were driving down the road and heard a beeping sound and then on the scrolling message display it read "Emergency Vehicle" sure enough, there was an ambulance about 1/4 mile in front of us. Can anyone explain how that worked?|||you think thats something my bmw beeps and then it says low flying aircraft/ near airport