Says the father of missing woman: "I know she's out there somewhere."

Detectives in Orange County, California, are baffled over the Valentine’s Day disappearance of 27-year-old Erica Alonso, last seen driving off in her car alone after an argument with her boyfriend following a night of partying.

“At this point we just can’t say that a crime has been committed,” Lt. Jeff Hallock of the Orange County Sheriff Department tells PEOPLE. “It’s a missing person case right now. All we have is that she was last seen around 4 o’clock in the morning and there has been no activity on her phone, bank account or on social media.”

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According to police, Alonso and her boyfriend spent much of the night at the Sutra Lounge in Costa Mesa, California, before she ended up making friends with another couple, who drove her and her boyfriend back to his apartment in Irvine, California, at 1:30 a.m.

The couple later told deputies that they left Alonso and her boyfriend at 3:30 a.m. after “they started to argue,” says Hallock.

When questioned about Alonso’s disappearance, her boyfriend “indicated that at 3:45 a.m. [she] left on her on her own and drove away,” says Hallock, adding that the boyfriend is “cooperating” with authorities.

Investigators believe that locating Alonso’s car [a four-door 2014 Honda Civic EX with California license plate 7FSS563] could be the key to finding her. “Her vehicle hasn’t been seen since she left her boyfriend’s house,” says Hallock. “But we are assuming she is with the car.”

Alonso had been known to “take off” for a day or two in the past, family members told deputies, “but there had always been phone calls or text messages,” adds Hallock. What makes her recent disappearance so troubling, he says, is that “everything just came to a complete stop on Sunday morning.”

At an emotional press conference on Feb. 19, distraught family members held out hope that Alonso might still be alive. “We just want her home,” said her sister Patricia. “We miss her so much.”