I could not get this option on my car and I really wanted to have it (in addition to the backup camera. I really think visibility is not great on the IS) but then the nearby dealers did not have a car with all the options I wanted (and I could not wait to have it). Now I am a bit puzzled as I was not aware it is a manual thing. I am surprised, to say the least, that it is not automatically switched on / off. I had this on many cars and usually when I get in reverse (never had it on the front bumper) it will stay quiet and as I approach something it will start beeping faster as I get closer.
And when you say it beeps when you have a person walking in front of the car this really surprises me as I thought this is somewhat intelligent function that can "tell" if you are driving or parking. What happens if you are on a stop light and get close to someone in front of you or someone in the back gets close by? Does it beep again? I am almost glad now I did not get this option!