An Army sergeant, missing for 26 years after serving in Vietnam and presumed dead by the Pentagon, has turned up in Georgia filing for Social Security benefits to the shock of his relatives in Hawaii and the amazement of the U.S. Army.

What happened to Master Sgt. Mateo Sabog, whose name appears on the Vietnam Veterans' Memorial, remained a mystery to much of the outside world yesterday as Army doctors began running the 73-year-old onetime transportation specialist through medical tests at Fort Gordon near Augusta, Ga. Army officials declined to disclose much of what Sabog had told them about his life since the Army lost track of him in Saigon in February 1970.

But they acknowledged being dumbfounded by news that Sabog had walked into a Social Security Administration office in Chattanooga last week and asked to start receiving benefits.

"We're welcoming him back as a long lost soldier who has returned," said Lt. Col. Bill Harkey, an Army spokesman. In fact, Army offices were abuzz yesterday with word of Sabog's remarkable reappearance, as more than one official remarked in interviews that the story had the makings of a made-for-TV movie.

Before releasing details of Sabog's strange disappearance, Army officials said they wanted to get a better fix on his mental condition and determine how willing he might be to talk to reporters. They also said no decision had been made about legal action against Sabog, although one senior officer discounted the possibility that the sergeant, whose active duty status is being restored, would be brought up on charges of desertion or anything else.

"No one in the Army is going to want to beat up on this old man," the officer said.

The last sighting the Army says it had of Sabog showed him in Saigon in February 1970 preparing to leave after serving there with the 507th Transportation Group. He was supposed to report to Fort Bragg, N.C., but never did.

"Fort Bragg had never been notified he was coming, so when he didn't show up in March, he wasn't missed," Harkey said.

Even the Air Force flight that carried Sabog out of Vietnam cannot be determined by Army investigators trying to retrace the sergeant's travels.

"The Air Force, in all its infinite wisdom, destroyed all the manifests of its flights," said an Army official. "We know he got out of Vietnam, but we're not sure exactly when."

Sabog could have passed through any of several way stations on his return to the U.S. mainland -- the Philippines, Japan, Hawaii. At some point on the trip, Army officials say, Sabog appears to have simply walked away from the military and ended up living in California with a woman.

"They stayed together for years," said an official, recounting from a preliminary debriefing of Sabog by Army investigators. "What precipitated him coming out of the woodwork was that the woman recently went into a nursing home or passed away. So he lost his support."

When Sabog showed up at the Social Security office in late February, coming down to Chattanooga from his small house in the textile manufacturing town of Rossville, he had no identification papers. But he produced the name of a brother in Hawaii who, when called by a Social Security Administration official, was startled to say the least.

The brother, Kenneth -- one of four Sabog siblings in Hawaii, according to Army officials -- had also lost track of Mateo since Vietnam. It was a letter Kenneth wrote to the Army in 1973, asking for information about Mateo's whereabouts, that first made the Army aware Mateo was missing.

Initially, the Army listed Sabog as a deserter. After another letter from Kenneth -- this one in 1979 to President Jimmy Carter challenging the desertion charge -- the Army convened a board of officers. Reviewing FBI reports and accounts from people who knew Sabog, the board concluded Sabog was not a deserter and presumably was dead.

"There was no reason to believe that an individual who had 24 years of honorable service, was retirement eligible and was returning from Vietnam would desert from the military," the Army said in a statement yesterday.

In 1993, at the urging of Sabog's siblings, his name was added to the Vietnam memorial, although -- at the family's direction -- with a small cross etched next to it, indicating missing rather than dead.

"Kenneth Sabog had corresponded with the governments of Cambodia, Laos and Vietnam, looking for news of his brother," said memorial representative Emmelene Gura, who escorted the Sabogs to the 1993 ceremony. "He was frustrated more than anything because no one could tell him anything."

The Sabogs plan to reunite with Mateo in Georgia next week. CAPTION: Mateo Sabog gets a haircut Thursday in Chattanooga, 26 years after he disappeared on his return from Vietnam.