The brown fish owl was spotted and photographed by a group of environmentalists in a field visit to the site in Sham Chung the middle of May.
It was seen flying and then landing on a branch of a tree as researchers from the Eco-Education and Resource Centre and Green Power were approaching a woodland area.
The owl, which is brown-yellow in colour with some black spots, is about 60cm tall. Its feet and lower legs are unfeathered, which allows it to wade in the shallows to catch fish without its plumage getting wet.
It is a protected species in Hong Kong and is considered globally vulnerable.
'It might have been scared by the noise of the researchers and therefore appeared before us,' said Ken Ching See-ho, a spokesman for the resource centre.
But the owl has not been seen since then.