It's Peter Jackson, one of the two survivors in the town according to the book.
Another silence, then Crane: "Sir!"
Crane: "You see him? The man in the white robe, walking across the street--"
Shawn: "I see him."
Crane: "He's just stepping over them like--"
Shawn: "He's coming toward us."
Crane: "Sir, look, I think we should get out of here, if you don't mind my--"
The next sound was a high-pitched scream, and a crunching noise. Transmission ended at this point, and Vandenberg Scoop Mission Control was not able to raise the two men again.
"Look at this," he said, pointing to a figure. It was a man in knee-length white robes, standing and looking up at the plane. He was an old man, with a withered face. His eyes were wide.
Burton was beginning to regain his composure now, after the initial shock of the town. He was beginning to think clearly. They walked up to the van, standing in the middle of the street, its lights still shining weakly. Stone reached in to turn off the lights. He pushed the stiff body of the driver back from the wheel and read the name on the breast pocket of the parka.
The man sitting rigidly in the back of the van was a private named Crane. Both men were locked
in rigor mortis.
He stopped. He was looking at the face of Shawn, who had obviously pitched forward hard onto the
steering wheel at the moment of death. There was a large, arc-shaped cut across his face,
shattering the bridge of his nose and tearing the skin.
He was an old man, with thin gray hair and a wrinkled, worn face. He wore a long nightgown that was smudged with dirt and yellowed with dust, and his feet were bare. He stumbled and tottered toward Stone. His chest was heaving with exertion beneath the nightgown.
"Who are you?" Stone said. But he knew: the man in the pictures. The one who had been photographed by the airplane.
"We won't hurt you," Stone said. "What is your name?"
"Jackson. Peter Jackson. Sir. Please don't hurt me."
So, long story short, one of the survivors is still wearing the white night-gown he was wearing when everyone suddenly died. Shawn and Crane saw him moments before the disease hit them, causing Shawn to convulse forward, smashing into the steering wheel (and possibly leading to some sort of a crash).