Tith Plok vanished again last weekend.

The 27-year-old Cambodian refugee took with him a friend's backpack stuffed with a pair of pants, his father's shoes, two shirts and a toothbrush, according to friends.

It was just about a year ago that Plok, the son of rice farmers from mountainous northeastern Cambodia, departed on a similiar journey, heading south on foot. Members of his family said the former Manassas brick factory worker had told them that he wanted to walk home to Cambodia. He turned up eight months later in a Salvadoran jail.

A publicized joint effort by an international refugee group, the U.S. Embassy in El Salvador, Rep. Frank R. Wolf (R-Va.) and an Arlington social worker brought him back to Arlington. But on Sunday morning, one day after returning from a two-month stay at the Northern Virginia Mental Health Institute in Fairfax, Plok left.

Some social and mental health workers said yesterday that he was a disoriented, traumatized man who, like some other Indochinese, has not adapted well to his new world.

"There are so few possibilities for these clients," said Dennis Hunt, who heads the Multicultural Council of the Mental Health Association of Northern Virginia, which is studying the problems of immigrants. "He sort of exemplifies a lot of the frustration we feel."

Hunt and others said the difficulty in Plok's case is that he is not ill enough to be hospitalized but requires some professional assistance he might not seek because of cultural barriers.

"This kind of notion of going outside the extended family for help, that's a very alien concept," he said.

Plok received a battery of psychological tests and was treated for hepatitis at the Mental Health Institute, sources said. According to the sources, there was no medical reason to keep him at the facility, which treats the mentally ill.

Last Friday Plok was released. An employe of Arlington Community Residences Inc., a private nonprofit mental health organization that had been helping Plok arrange for housing, took Plok to his father's apartment. Plok had wanted to be alone, away from noisy children, and had agreed to that arrangement, said Jose Campos, of the Arlington group.

The group had collected $550 from other local relief agencies for a deposit on an apartment and the county gave Plok $243 more to help out.

On Saturday, Plok, his family, Campos and an interpreter met at the Arlington apartment to discuss a snag in the housing arrangment: the apartment he was to move into would not be available until Monday.

"He looked happy, he was smiling," said Campos. "There was no indication that he wanted to run away. We were going to help him find employment. We were trying to help him become independent."

By Sunday morning, however, Plok was gone. His history has been recorded in the National Crime Information Computer, where he is listed as an "endangered missing person."

Yonnara Keng, a Cambodian social worker in Arlington who knows Plok and his family, said his family is upset but does not know what to do. "His uncle said Plok was okay, his appearance was good. His face is brighter . . . . They thought he just went out for a walk, but he never came back. The family couldn't believe it. They just couldn't believe it."