THE United States had a secret MIA office in Hong Kong until this weekend, the Sunday Morning Post has learned. The office, responsible for interviewing witnesses in the Vietnamese camps, has been relocated to Bangkok. The move comes despite the territory's Vietnamese having proved a valuable source of information leading to the identification and return of a number of remains to their families. The northern Vietnamese are regarded as a better source of knowledge because most planes were shot down there and the people seem to have better memories - perhaps because they suffered more. Some of the camp people were children at the time and can remember watching their fathers and villagers shooting at aircraft from the jungle. Amazingly, they can recall exactly where the planes fell and have provided detailed directions 20 or 30 years later. Other sources are former Viet Cong, communist party policemen and simple farmers who were protecting their land. Some have even carried MIA remains on the boats from Vietnam, and held on to them for years. More recently the Vietnamese have used MIA bones, dog-tags, driving licences and credit cards as bargaining chips to gain refugee status. A source at the US embassy in Bangkok said: ''The communist police were the hard of the hard, and also the most corrupt. Many were exposed after the war and fled before they were caught. ''They went to Hong Kong and got rejected as political refugees. So they ask to see the MIA guys there and hand over information and fragments of bone. ''Then they call their liberal lawyers and tell them they've spoken to the US and would now face certain persecution if they were sent home. Whammy . . . they're classed as political refugees.''