Nine years after 650 American prisoners of war came home from Vietnam, at least one mystery remains: are any more POWs still alive half a world away, held captive by Vietnam?

And, if so, what, if anything, would Vietnam stand to gain by keeping them hostage?

For years, such questions have been fueled by hundreds of Vietnamese refugees who claim to have seen emaciated American prisoners in remote camps from Vietnam to Laos. About 500 relatives and friends of POWs and others still missing will trek to Washington Friday for National POW-MIA Recognition Day ceremonies. Among them will be Dennis and Alice Duckett from Ringgold, Ga., who live with the memory of a son who survived the crash of his Air Force spotter plane over Laos in 1970.

Radio contact was made with Lt. Thomas Duckett, 24, after the crash. And an American prisoner fitting his description was identified in Laos in February, 1974, according to classified documents obtained by his family under the Freedom of Information Act.

Even though the Pentagon pronounced him to be presumed killed in action in 1979, Alice Duckett believes her only son is still alive. "Call it mother's intuition," she said. "They're going to have to give me more proof."

The possibility that Duckett and others might still be held prisoner is kept alive by men like Nguyen Duc Yen, a former Viet Cong propaganda chief in the prime minister's office who escaped via Hong Kong with his family in 1979.

He told Parade magazine, in an article which will appear Sunday, that he saw at least 30 to 40 American pilots alive in North Vietnam in 1973, two months after the North Vietnamese said they had freed all American POWs.

Yen, now living in Switzerland, said the Americans were doing "hard labor," kept alive as bargaining chips in a game of diplomatic poker.

"I wouldn't doubt he was telling the truth in 1973," but there is no proof the men are alive today, said a Defense Department spokesman who asked that his name not be used.

Still, reports by refugees of live Americans--"live sightings," as they are called--keep trickling into the downtown Washington office of the National League of Families of American Prisoners and Missing in Southeast Asia.

The league advertises for leads in a California-based, weekly Vietnamese newspaper with worldwide circulation, forwarding all tips to the Defense Intelligence Agency.

The DIA is investigating 426 cases of alleged live sightings of Americans, including three this year. "The attitude is that you have to believe there is at least one guy being held against his will just to maintain a level of vigilance in checking these things out," the spokesman said. "Unfortunately, we haven't been able to prove that's the case."

Defense officials argue both sides. On one hand, there are 2,456 ser- vicemen unaccounted for. At least 200 are air crewmen like Duckett known to have landed or parachuted to safety as late as 1973.

Marine Pfc. Robert Garwood, convicted last year of collaborating with the enemy during 14 years in captivity, is a recent example of one who survived.

"So, you can argue that it is statistically probable a healthy man would be alive today, if he wasn't captured or killed on the spot," the spokesman said, adding that some French POWs were reportedly held until 1977, 23 years after the fall of Dien Bien Phu.

On the other hand, few refugee accounts have jibed on locations of prison sites, and officials say they believe the refugees could easily confuse American POWs with other Caucasians roaming the Vietnam outback after the war, like Scandinavian laborers or Russian advisers.

The Pentagon debriefed the 650 POWs released in 1973, and believes it has identified every American held in a Vietnamese prison, officials said. And the State Department remains officially baffled at why Vietnam would still keep Americans hostage.

"Our view is that it would not be to Vietnam's advantage to withhold them," a State Department officer said. "It's not only upsetting to the families, but has gotten Vietnam a great deal of negative publicity."

At its 13th annual meeting in Washington, the league plans to deliberate live sighting reports of varying reliability that have been investigated by the DIA.

A number of refugees have passed at least one lie detector test administered by military investigators, officials said, but no one has passed a follow-up test. Yen flunked several polygraphs.

He provided investigators with information about American civilians imprisoned in Chi Hoa prison after the fall of Saigon in 1975, according to a statement issued by Defense Department today. But polygraph tests indicated he "fabricated" information about sighting downed American pilots, as well as personal background data.

Last year, a CIA-backed band of Laotian mercenaries were unable to confirm the presence of American POWs who intelligence data suggested were being held at a jungle site deep inside Laos. But President Reagan has vowed to take "appropriate action" should hard evidence show Americans are being held.

"I assure you that actions to investigate live sighting reports receive and will continue to receive necessary priority and resources based on the assumption that at least some Americans are still being held captive," Reagan wrote the league in a June 30 letter. "Should any such reports be proven true, this government will act decisively to ensure their return."