What evidence is there that American POWs were left behind in Vietnam?

[Measure_for_Measure]

I understand MIA means they haven’t recovered the body. About half the original MIA figures were known to have been killed in action:

From the link in the 2nd post:

When Franklin published M.I.A. in 1992, there were still 2,273 “unaccounted for” Americans from the Vietnam War; according to the Department of Defense’s POW/MIA Accounting Agency, there are still 1,587 unaccounted for as of this year. Even the original total of missing Americans in the Vietnam War is considerably smaller compared to previous conflicts; as of September 13, 2019, the Department of Defense still lists 72,661 persons as “POW/MIA” from World War II. And as Franklin points out, at least half of the unaccounted for at the time of publication were known to have been killed in action — it’s only because their bodies were not recovered or unidentifiable that they were classified as “missing.”

A 1991-93 congressional investigation by John McCain and John Kerry found, “no compelling evidence that proves that any American remains alive in captivity in Southeast Asia.” Currently the Department of Defense spends $110 million per year tracking down MIAs. The majority of the 150 or so newly identified each year are from wars before Vietnam. Vietnam War POW/MIA issue - Wikipedia