Seems to me he might have noticed in his mirrors the car under his bumper as it was not totally centered, but maybe not.
As far as noticing the impact, when I was younger and working in our warehouse I was talking to a truck driver one day. Told me how he was at a light waiting to make a right turn. He was actually set across two lanes, with his signals on with the tractor part in the left hand lane so he could cut the corner with a long trailer. What he didnt notice was a woman pull a car up next to him in the right lane. When the light went green he pulled forward and made the turn and the trailer "thumped" as he called it and he thought he had just caught the curb with his rear wheels on the trailer. What actually happened was the woman pulled forward as the light went green and he pulled the trailer across and crushed the entire front of her car. He found out about it when someone pulled up next to him down the road and told him what happened.