There is a long established thread documenting the first arrival of Indian cuckoos on Lamma, and it is usually in the first week of April. Now is almost impossibly early for it to be these migrants.
See
viewtopic.php?f=46&t=637However, there is the koel which calls incessantly in spring and sporadically throughout the winter on warm spells, especially early in the morning. I am not in HK, but I would guess it might be one of these, giving the koo-ee call, rather than the four note 'my momma told me...' of the Indian cuckoo.
Of course if it sounded like an ENGLISH cuckoo, that's another story altogether, especially if it also whispered cryptic instructions to go out and do something radical...