When the UK handed back HK, the Chinese who are nothing if not wiley, understood that they needed to maintain intelligence, surveillance, and some kind of institutional knowledge of the various organised crime groups, certain individuals with borderline business interests, that sort of thing. They offered the British police officers houses, stipends, and other incentives to stick around and clue-in the incoming crop of officials, domestic intelligence officers, and cutouts/go-betweens. Something of an untold story. Would make a great streaming series.