A metal object that crashed through an 82-year-old man’s windshield and killed him as he drove on Highway 101 last week in the South Bay was a water cap that somehow detached from a California Department of Transportation truck going the opposite direction on the freeway, officials said Monday. The Caltrans water truck, used to spray down dust and debris at construction sites, was heading south on 101 in between assignments, a spokesman for the department said.