Michael Beasley tells San Diego, California, TV station CBS 8 that on the morning of Saturday, June 28, after Corwin left her at home in Twentynine Palms, reportedly for a solo day trip to Joshua Tree National Park, he saw her meet a man, exit and lock her blue Toyota, and then climb into the man's red sedan.
"They were just sitting and chatting," said Beasley. "If she wanted to get out of the car and say, 'I don't want to be in this car,' she could have."
The pair then drove off together in the man's vehicle, he said.
"When I came back a half-hour later, just the [blue] car was sitting there," said Beasley. "Next thing, I hear the weekend goes by and Monday morning the sheriffs are there looking for this girl because she ain't home yet."
Meanwhile, warrants made public this week reveal investigators sought permission to search two apartments on the Marine base that is home to Corwin, whose 20th birthday was Tuesday, and her husband, Marine Cpl. Jon Corwin.
The warrants also mention a 2013 blue Toyota sedan with California license plates, and a dark-colored Jeep Cherokee with Alaska license plates.
In each case, the otherwise identical documents indicate authorities are looking for items such as DNA and hair samples, latent prints, anything "soiled with bodily fluids," and electronic devices, including computers.
Investigation UnderwayWhile acknowledging "it's an active investigation that, at some point, could turn into a criminal investigation," Cindy Bachman, a spokeswoman for the San Bernardino Sheriff's Department, tells PEOPLE: "We haven't located any solid evidence that has established this as a criminal investigation. We haven't identified anyone as a suspect or a person of interest."
"At this time," she adds, "it's a missing-person investigation and a search for a missing person."
"We need to let the investigators continue to do what they're doing," she says.
Corwin's hopeful family has established a Facebook page directing anyone with information to call 760-366-4175.
San Diego, California News Station - KFMB Channel 8 - cbs8.com