The box truck was parked beside a quiet sidewalk in the West Loop. A few people stopped, glancing curiously at the image flashing on its rear door: a young man, his expression resolute but tinged with sadness. Below his figure was a message in bold letters: “Chicagoan kidnapped by Hamas.” “He’s always up there,” said the truck’s driver, Jeremiah Smith, 27, as he stared at the image. “I always know that.” Twelve hours a day, six days a week, Smith drives the truck through the Chicago area, raising awareness for the scores of Israeli hostages held captive by Hamas. But...