Today I passed by "the scene of the crime" and discovered what the "white stuff" was: stones painted white on either side of the sidewalk/footpath just there. So, not debris but decoration.
This evening we visited James, 32, and Kim, 26, told them the story, and asked their opinion of what might have happened. We thought they might have a better idea than we about their peers' behavior, and they did. The soundness of a theory is measured by how many of the data it explains and as this theory neatly explains it all, I pass it on to you.
The young man and the young woman would have been out drinking on the Saturday night. He would have been showing off for the girl by driving too fast on the way home and would have missed the s-curve right there and have hit the curb. Hence, no shattering glass or sound of smashed metal, just the too-rapid deceleration without any squealing of tires and the very loud, dull thud.
The young woman would have been furious and have started screaming at the drunk driver. Women are never impressed by speeding, regarding such showing off as childish, and James and Kim tell us that the local women, when drunk, are extraordinarily obnoxious anyway.
The young man would have panicked and have run to his father, the older man I first met, who must live very near us. A bit later, he ran to his dad again to be sure the police were not coming. This is why his dad had not called them, and why my involvement was unwelcome.
The car probably belonged to the father, which would explain why he then went to have a look at the damage and was so upset, and why he had the key.
Meanwhile, the young man and woman were walking home to his parents' house.
In the end, one of them simply summoned a tow truck so that by dawn there was no sign of any accident and the car's two drunken occupants were safely away from the scene and sleeping it off.