San Diego County Sheriff Bill Gore said the riders reported seeing the two in the Cascade area 70 miles northeast of Boise on Wednesday but didn't know they were being sought until seeing news reports.
The 16-year-old girl, Hannah Anderson, did not appear as if she was being held against her will, Gore said, without elaborating. Both people appeared to be healthy.
Authorities throughout the West have been looking for the teen and family friend James Lee DiMaggio, 40, since the bodies of the girl's mother and an unidentified child were found Sunday at DiMaggio's burned home 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 positive identification hasn't been made.
DiMaggio had bought camping gear a few weeks ago and investigators said they believe his flight was planned.
DiMaggio was a close family friend. The children's father, Brett Anderson, has described him as a best friend who was so close to the children they 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.
Gore said the four horseback riders were on an outing in an Idaho wilderness area when they met and chatted with the man and teenage girl. The witnesses noted that the pair had light camping gear even though it was an extremely rugged area.
They didn't report it until later.
"They'd seen the story on the national news and put two and two together," then contacted authorities, Gore said.
The people spotted were wearing backpacks and had a tent.
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 release details of the conversation.
Around 8 a.m. Friday, DiMaggio's blue Nissan Versa was discovered 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.
The car will be examined by bomb and arson experts to ensure it isn't booby-trapped.
Police previously warned that the car may have been abandoned and rigged with explosives. Evidence found in the rubble of the burned home suggested DiMaggio may have fled with homemade explosives.
DiMaggio, who is wanted on suspicion of murder and arson, is a telecommunications technician at The Scripps Research Institute in San Diego.
AP writers Bob Jablon and John Antczak contributed to this report from Los Angeles.