Dr. Doogie said:
One important aspect about this couple: If they returned one month after the inital abduction attempt and did take Anna (which is the most logical explaination for Anna's disappearance), then they either lived somewhere within a half-day's drive of Half Moon Bay or were in the area for an extended stay between December 1972 and January 1973. It would make sense that they were close enough to make two trips a month apart, but lived far enough away that the local media coverage of would not raise suspicion as to why they sudden had a daughter. The San Francisco media market would cover the entire Bay Area from Santa Rosa to the north, San Jose to the south and Livermore to the east. This is why (in my mind) the central valley of California is the most likely area that they would be from (from Sacramento to the north, down to Fresno or Bakersfield to the south). The central valley would be within either the Sacramento market or their own media markets, and an abductor would thus avoid the glare of public scrutiny.