On Aug. 28, Leah Rose Altmann left the Los Angeles apartment she shared with a roommate. Carrying only a backpack, she didn’t say where she was going. She left most of her possessions behind, including her clothing and makeup.

She hasn’t returned. About a month later, police in L.A.’s North Hollywood neighborhood spotted her outside a Panera Bread. It was the last time she has been seen.

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Altmann, 27, was an avid user of social media, but her accounts have now gone dormant. She didn’t call her mother on her birthday in November — which family members say isn’t normal.

“She just vanished,” her father, Paul Altmann, tells PEOPLE. “We have no idea where she is or what she’s doing. Everyone is very worried about her, because she just wouldn’t vanish for so long.”

Image zoom Palm Beach County Sheriff's Office

“She’s 27, but she’s only 4-foot-9-inches tall and 90 lbs.,” Paul continues of his daughter, who is fond of wearing boots and large sunglasses. “She’s got blonde hair and blue eyes. She looks like she’s a teenager. We’re hoping for the best, but we just don’t know where she is. ”

A musician who grew up in South Florida, Leah moved to L.A., where she worked in the sound and audio industry. She would often land a gig and vanish for several days. But “she has never been gone for this long,” Paul says. “And she’s not on Facebook, Twitter, Snapchat, Instagram … that’s not like her.” Calls to Leah’s phone go straight to voicemail.

The months-long mystery took a foreboding turn on Tuesday when the Palm Beach County Sheriff’s Office in Florida tweeted Altmann’s photo along with a message for the public: “This local girl may possibly be a victim of Human Trafficking.”

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For Paul, the idea that his daughter could be a trafficking victim is devastating.

“I hope it’s not true,” he tells PEOPLE. “The best-case scenario would be that she’s doing whatever she wants to do. She’s doing a gig and not paying attention to the fact that people are looking for her.”

Despite that hope, Paul admits that his mind sometimes goes to darker places. “I want to believe that she’s okay and that she voluntarily went off the grid,” he says, “but when my eyes close at night, that’s not what I see. I think about the bad things that may have happened to her.”

“There are so many explanations,” he adds. “Could it be drugs, alcohol, mental illness? Sure. Could she have been kidnapped? Yes. She’s this little girl, and I think about what could have happened to her. But we are holding out hope.”

Although Leah was living in California, she could now be anywhere. Authorities in both California and Florida are searching for her, but she could be somewhere in between — practically the entire country. She often uses public transportation.

“I want her to know that she’s loved,” says her dad. “There are people who are looking for her. We don’t understand why she is off the grid, it doesn’t make sense. I just hope she can let us know that she’s okay.”