Many thanks to John and George.
This is another photo of the bird but sorry that it still cannot show the head of the bird clearly.
There is a large fruiting tree at Fung Yuen and a lot of bulbuls and some thrushes feeding on it. From the area of this fruiting tree, there is a narrow path leading to a large area of grass. This warbler was found near the end of the narrow path, near the large area of grass.