I read your post and wondered if your scenic has the Renault emergency braking assist system fitted ?
From the renault website
"The emergency braking assistance detects emergency conditions from the speed with which the driver hits the brake, boosting the braking power (which is controlled by ABS) to reach maximum pressure almost immediately."
( Not meaning to boast ) If I stamp on the brake pedal above 40mph the car stops on a six pence and the hazard warning light automatically come on to warn people behind. I am not sure how this system is calibrated but it may be worth thinking about getting yours checked if it's fitted.