ajohn

According to the Ripley Guide to South Asia (Rasmussen and Anderton) Lesser Cuckoo parasitises Cettia and Phylloscopus warblers.
Of these, the only species potentially breeding in HK is Brownish-flanked Bush Warbler, but the population of these seems small, so I think it is unlikely that Lesser Cuckoo breeds regularly in Hong Kong

ajohn

No doubt Lesser Cuckoo could use alternative hosts. Certainly common cuckoo has been recorded parasitising many different species. Personally I would have thought tailorbirds/prinias were unlikely due to their nest shape (although Plaintive does use these as hosts).  Mountain tailorbird (which is supposedly related to Cettia) may be possible but even so I think it is unlikely that such a rare migrant would successfully manage to find a mate and a suitable alternative host.

I agree with Mike that it is unlikely that a regular breeding population has been overlooked - more likely this is a late migrant that has stopped en route to China.