The four riders reported seeing the two in the Cascade area, 70 miles northeast of Boise, around noon Wednesday. They didn't report it until later, after seeing news reports and realizing the pair was being sought, San Diego County Sheriff Bill Gore said.
The girl, Hannah Anderson, did not appear as if she was being held against her will, Gore said, without elaborating. She and the suspect, James Lee DiMaggio, 40, seemed healthy.
The car, a blue Nissan Versa, was found Friday morning, covered in brush off a road about 5 or 6 miles from the spot where the man and girl had been seen. The license plates had been removed but the vehicle identification number matched that of the car being sought, Gore said.
Bomb experts planned to examine the vehicle. Police previously warned that the car may be rigged with explosives.
Authorities throughout the West have been looking for the teen and DiMaggio since the bodies of the girl's mother, Christina Anderson, 44, and an unidentified child were found Sunday at DiMaggio's burned home near the Mexican border about 65 miles east of San Diego.
The unidentified body is believed to be that of Hannah's 8-year-old brother, Ethan. The body was badly burned and definitive identification hasn't been made.
DiMaggio was close to the family. The children's father, Brett Anderson, has described him as a best friend and said his children thought of him as an uncle.
Authorities have said DiMaggio had an "unusual infatuation" with the 16-year-old, although the father said he never saw any strange behavior. If he had, he said, "we would have quashed that relationship in an instant."
DiMaggio, a telecommunications technician at The Scripps Research Institute in San Diego, was planning to move to Texas and invited Christina Anderson and the children to his home last weekend to say goodbye, said Christopher Saincome, Anderson's father.
It's unclear how the two were killed, though police believe the crime was planned. Gore noted DiMaggio bought camping gear a few weeks ago.
Brett Anderson said his friend is an outdoorsman.
"He was very interested in hiking and camping," Anderson said. "Whenever he had the chance and had the funds he would go on a hiking trip."
Cascade is an outdoorsman's paradise. Perched along the southeastern shoreline of the Cascade Reservoir and The West Mountains, the town is a popular summer getaway for hikers, campers and kayakers.
About 25 miles away is the western boundary of the Frank Church River of No Return Wilderness, a vast and rugged 2.3 million-acre preserve in the heart of Idaho. Because of its wilderness designation, the preserve is not open to motorized vehicles and traffic is limited to hikers and horseback riders.
DiMaggio and the teen were hiking when the horseback riders encountered them, Gore said. The riders noticed the pair had light camping gear even though it was an extremely rugged area.
The riders "did seem to think the two of them were out of place in that area," Gore said.
The sheriff said the riders chatted briefly with the couple but he didn't reveal details of the conversation.
AP writers Bob Jablon and John Antczak contributed to this report from Los Angeles.