One moment last November, Imelda Calderon, then 57, was standing next to her husband, Juan, at a Target store in El Paso. Juan Calderon looked down for a few seconds, fumbling with his credit card at the register. “And then all of a sudden I turn around, and she wasn’t there,” he said.

His first panicked thought, he said, was that he might never see her again. His next was a prayer that she please not end up across the Rio Grande in Juárez, a city with serious crime problems.

Wandering is a common behavioral effect of dementia: An estimated 60% of people with the condition will wander at some point, according to the Alzheimer’s Association. Patients can become confused and disoriented, forget where they are going or where they live, and stumble into harm’s way.

And while wandering is one of the biggest worries facing caregivers, it is especially so near an international boundary, where potentially unfamiliar hazards may loom.

For Alzheimer’s and dementia patients and their caregivers, living on the border comes with unique challenges, according to Monica Moreno, senior director of care and support at the Alzheimer’s Association. “It certainly makes it more complicated,” Moreno said. “There’s more risk of something to happen.”

When Calderon’s adult children discovered she was missing, they rushed to catch flights to El Paso, and friends and relatives on both sides of the border mobilized to look for her, with two motorcycle gangs even joining the search. At the same time, Juan Calderon said, Imelda was walking roughly 2 miles to a Food King grocery story about a mile north of the border, where he believes she hitched a ride with a good Samaritan heading across the international bridge.

The story he tells is dotted with “probably” and “I think,” as he tries to fill in the blanks in her journey that still puzzle him.

Imelda Calderon grew up in Juárez, he said, so she may have gone there thinking it was still her home. Once there, she met four sisters, who may have reminded her of her own sisters. The women took her with them, got in touch with her relatives in Mexico after seeing social media reports about the search, and cared for her until the relatives came to pick her up.

Juan Calderon said their children blamed him for what had happened. “They were mad at me because they thought I wasn’t paying attention to her,” he said. “I wasn’t because I turned around. But I wasn’t planning on losing her.” Calderon said his wife had never wandered before. Now when they go out, he said, he never lets go of her hand.

How often do memory-impaired patients wander across the border? A spokeswoman for Customs and Border Protection, Jacqueline Wasiluk, said the agency does not keep a formal tally, “but anecdotally, it’s very rare.” Even so, local news outlets have reported at least four incidents in the San Diego area since 2016.

When people with Alzheimer’s or memory impairment do wander across the border, days may pass before they are found and identified. In 1986, a San Diego man was found “shoeless and suffering” under a bridge in Tijuana two weeks after he had wandered from home, The Los Angeles Times reported.

Alzheimer’s disease can manifest itself differently from patient to patient, and the tendency to wander is not predictable. Moreno said there were things that caregivers and families could do to minimize the risk, like maintaining regular routines; keeping the patient’s living environment well lit and easy to navigate; making sure the patient always wears or carries identifying information; using a tracking device; and taking special care when in stimulating or tiring environments like airports or shopping malls.

Most cars from the United States are allowed to enter Mexico with little or no review. Once a wanderer has crossed the border, though, the foreign surroundings may seem comfortingly familiar – or frightening and disorienting.

Evelyne Delorme’s most recent wandering episode started with a tiny window of opportunity.

Her husband was asleep. Her son was in the shower. The car keys, usually kept hidden, had been left out in the family room.

Delorme, 71, a cellular and molecular biologist with a Ph.D. from Cornell who has early-onset Alzheimer’s, took the keys and got into the silver Toyota Camry that she had not driven in at least a year. Four right turns and one left turn put her on Interstate 5, headed south. She kept going for 30 miles, finally stopping when she crashed into another car — in Tijuana.

“We were very surprised,” said her son, Brian Fish. “I thought she would head north. We didn’t think of Mexico at all.”

For Delorme to wind up in Tijuana that day in January required a confluence of relatively unlikely circumstances: Her dementia-driven impulse to wander had to coincide with a caretaker’s momentary lapse, and then she had to happen upon a path to the south, rather than east, west or north.

When Delorme’s family discovered that she had driven away, her husband, Leonard Fish, contacted the police while their three grown children began driving around looking for her. They tried locating her by tracking the iPhone she carried, but the phone was not online; they later learned its battery had run out.

Fish said his wife headed for Mexico by chance. “She just happened to get on the freeway, and if you’re going south on 5, that’s where you end up,” he said. “You’d have to make a decision to turn off, not to end up at the border.”

Chance came to her rescue when she got there, he said, in the form of the man whose car she hit. He treated her kindly and took her to the port of entry to get assistance from officials there. “That was a remarkable coincidence in my mind,” Fish said. “She ran into somebody with morals, and he helped her. It could have been way worse.” She was back with her family that night.