It appears there were rumors of more bodies, and with new forensic capabilities, I guess they decided to go looking. That was my take from this article,
Experts Find Possible Evidence of More Manson Family Murders
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For years, rumors have swirled about other possible Manson family victims — hitchhikers who visited them at the ranch and were not seen again, runaways who drifted into the camp then fell out of favor.
The same jailhouse confessions that helped investigators initially connect the band of misfits living in the Panamint Mountains to the gruesome killings that terrorized Los Angeles hinted at other deaths. Manson follower Susan Atkins boasted to her cell mate on November 1, 1969, that there were "three people out in the desert that they done in." Other stories surfaced. In the absence of bodies, they were forgotten......
....Last month, equipped with cutting-edge forensic technology, the investigators assembled in the ghost town of Ballarat for a 20-mile ride in all-terrain vehicles to the ranch.
The team included two national lab researchers carrying instruments to detect chemical markers of human decomposition, a police investigator with a cadaver-seeking dog, and an anthropologist armed with a magnetic resonance reader....In 1987, I rode a 5 day endurance horse race in Death Valley, and we rode right up the road past the Barker Ranch. It felt spooky then, and now the hair on my neck is rising again. The air all around me then just felt weird. Manson is truly a monster, and so are his followers.