“We tend to experience our identity as a thing, as if it’s a constant,” added Dr. Loewenstein, who is medical director of the trauma program at Sheppard and Enoch Pratt Hospital in Baltimore, and has treated five patients with dissociative fugue. “But it’s a lot less stable and has less unity than we want to believe.”

Image Hannah Emily Upp's rescuers, from far left, were Capt. Christopher Covella, Ephriam Washington and Michael Sabatino with Brian Brennan, a pilot. Credit... Michael Oates

Travel is a defining characteristic.

“People have been known to not only travel across cities or countries, but also across continents,” said Dr. Philip Coons, a professor emeritus of psychiatry at Indiana University and the author of a book on the subject. “The explanation behind the fugue is that the person is running away from a bad situation, from a bad marriage or a bad financial situation.”

The Missing Chapter

When Ms. Upp failed to return to her apartment after four days, her roommates contacted the police. After a week with no word, and fearing that she had been a victim of a kidnapping or another violent crime, her friends and family posted messages on blogs and started a Facebook page called “We’re Not Giving Upp (on Hannah),” which was dedicated to tracking her down. Accompanying many postings was a photograph of a smiling young woman with warm hazel eyes, glossy brown hair and a white rose tucked behind one ear.

Despite the optimistic tone of the postings, her family was frantic.

“At first, you try to come up with any kind of possible theory that could provide a simple, harmless explanation of where she might be,” her brother, Dan Upp, said from Japan. “But considering the circumstances, you really can’t convince yourself that any of them are feasible, and you’re left with the unavoidable conclusion that something is very wrong.”

Ms. Upp credits the police with helping her piece together what happened during the missing weeks. Though details like where she ate and slept remain elusive to her, security camera footage and conversations with police detectives have provided some clues to the where if not the why.

According to police reports, Ms. Upp spent a lot of time in places like Riverside Drive, “where if you’re in running gear, no one’s going to look at you twice,” she said. When she revisited Riverside Drive after leaving the hospital, Ms. Upp said, “it seemed to make sense to me. Not only is it one of my favorite places, but there’s something soothing about the sound of water and just not feeling trapped in the concrete jungle.”

Ms. Upp’s doctors have helped her make sense of other clues, like her stops at the Apple store, where she was seen both checking her e-mail and speaking with a fellow Pace student.