Erica Melissa Alonso of Laguna Hills hasn't been seen or heard from since about 3:45 a.m. Sunday.
Alonso's 28-year-old sister choked back tears as she spoke to reporters.
"Erica is my sister, my best friend,'' she said. "It's very unlikely for her not to contact us ... Please, you guys, if anybody has seen anything, knows anything, has heard from her, thinks they've seen her, her car or heard anything, please call (investigators).''
Patricia Alonso added, "All information is very, very important, and we just want her home. We miss her so much.''
The missing woman's father said the family has "hope that she is still alive. We need your help. Please, if you see anything or know anything, help us,'' Isaac Alonso said, describing his daughter as "responsible'' and "very nice.''
Margarita Alonso said her missing daughter "was always a good girl; she was always in contact with us. That's why we started suspecting (something was wrong) when the next day, and the following day, she never called us or texted us or anything.''
Erica Alonso went out on a date with her on-again, off-again boyfriend on Saturday night, and they met friends at the Sutra nightclub in Costa Mesa, according to Orange County sheriff's Lt. Jeff Hallock.
Patricia Alonso said her sister met the boyfriend, who is in his 40s, for a lunch date that went into overtime. The sisters had planned to spend Valentine's Day night together watching "cheesy movies.''
She said she found out her sister was at Sutra when she received a Snapchat update from her.
The missing woman "befriended'' another couple at the club and the four left together about 1:30 a.m. Sunday to continue partying at Alonso's companion's home near Culver Drive and Deerfield Avenue in Irvine, Hallock said.
Alonso and her date took a cab to Sutra, and she talked her new friends into giving her and her date a ride to his home, where they continued to party.
But Alonso and her boyfriend got into an argument and the other couple left about 3:45 a.m., Hallock said.
Alonso left her boyfriend's home about 15 minutes later, and there has been no trace of her since, Hallock said.
"There has been no activity on Miss Alonso's bank accounts, social media accounts, her cell phone accounts,'' Hallock said, adding that was "very concerning'' to investigators.
Investigators called on the public to keep an eye out for Alonso's white 2014 Honda Civic EX, license plate 7FSS563.
"We believe she is with that vehicle,'' Hallock said.
There is no indication Alonso was depressed or had any other issues, he said.
Investigators sought out the woman's boyfriend because he appears to be the last person to have seen her, Hallock said. So far, everything he has told detectives has "checked out,'' the lieutenant said.
"We're still pursuing this as a missing persons investigation,'' Hallock said, noting there's no evidence so far that a crime has been committed.
Media coverage of the investigation helped detectives get in touch with the "mystery couple'' that Alonso met at Sutra, and they "corroborated'' what investigators already knew about the case, Hallock said.
Alonso and her boyfriend had dated for about two years before they broke up, according to her sister, who said the two continued to see each other and were working on their relationship.
Alonso, who lives with her mom and sister, is described as Latina, about 5 feet 3 and 110 pounds, with long brown hair and brown eyes, Hallock said.
Anyone with information about her whereabouts was urged to call (714) 647-7000, or (714) 647-7055.
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