Parking sensors and rear-end collisions

My wife just had a peculiar accident (fortunately without any damage) in our MS. Although she seems to have been responsible, it seems like something that could be avoided with a minor tweak to the car's software.

She was stopped at a light on a slight uphill gradient, with the hill-hold keeping the car still. She accidentally hit the brake pedal lightly. The hill-hold released, and the car rolled backward to hit the car behind her--fortunately at low speed.

She says that she didn't hear any beeps before this happened. I guess this makes sense. I think the rear parking-assist chimes are selectively active when the car is in reverse. You probably wouldn't want the car to chime when somebody pulls up close behind you at a red light. So suppressing the chimes when the car is in drive is logical.

If this is correct, perhaps the system could be improved by having the rear warning chimes go off when the car is in reverse OR actually moving backward. (Without creep, the car rolling backward while in drive is a more-common possibility than with ICEs.)

I haven't had this vehicle for very long. Do I understand the way it works correctly? Does this logic seem sound?