Lass was last seen in South Lake Tahoe, California on September 6, 1971. She worked as a registered nurse at the first aid station at the Sahara Hotel-Casino and her last logbook entry was timed at 1:50 a.m. Lass's car was found parked near her apartment in Stateline, Nevada, but nobody saw her leave the casino.

The day after Lass's disapperance, an unidentified male called her place of employment and her landlord to say she would be absent due to a family emergency. Lass was not actually having a family emergency at the time. The caller has never been identified. All of her personal belongings were left behind at her home, except her purse and the clothes she was wearing the night she vanished.

A postcard purportedly from the serial killer nicknamed the Zodiac implied that he had killed Lass. The Zodiac sent many written messages to newspapers and police claiming responsibility for disappearances and murders, but Lass's abduction does not fit his profile and the postcard was later found to have been forged by a police investigator. The Zodiac has never been identified. He is still considered a possible suspect in the disappearances of Robin Graham and Jeannette Kamahele, however.

Lass's case remains unsolved. She is missing under suspicious circumstances.