SPECIAL Duties Squad officers swooped on an unlikely illegal gambling den last night, arresting 24 people in a Western flat - almost all of them Thai maids. The flat in Third Street, belonging to a Thai woman, had been under observation but police admittedthey were not sure what they would find. The gamblers, who had been betting on a Thai card game, were taken in for questioning and more than $10,000 in cash was confiscated. ''It's one of the most unusual gambling cases we've seen,'' said Inspector Adrian Whelan.