If the estimates of her time of death are fairly accurate, then we have a scenario where a woman is kidnapped, kept alive for 8 to 14 days (18th to 24th December), before being found 4 to 10 days later. It is highly unlikely her dead body had been on Stinson Beach, unnoticed for several days. So had it washed up recently, or had the killer dumped the body on the beach shortly before it was found, having stored it for several days? There are two viable options: either she was deliberately placed on a beach somewhere near Bolinas Lagoon by her killer, or she was thrown into the water at a secondary location and was carried by the currents to the location she was found on December 28th 1969.
Associated articles: A Very Unlikely Zodiac Murder and Did Zodiac Call on January 4th 1970?