I've used the search feature but I couldn't find any substantial answers to this question.

To explain further, I was reading recently about a memo sent by a Vietnamese general after the war saying something about 1,000+ American PoWs still in captivity. I know by now that many, if not all, of them are dead (starvation, disease, execution, etc.,) but what I want to know is whether or not historians have reached a general consensus on:

a. whether a significant number of prisoners of war were left after Vietnam, and

b. if so, did the U.S. government really try to conceal this, or was the evidence truly lacking?