John Holmes

Lack of a wing-bar makes me think of Grey-crowned Warbler (S.tephrocephalus) or Plain-tailed Warbler (S.soror).  

The "commonest" Seicercus warbler in Hong Kong is Bianchi's Warbler (Seicercus valentini)which usually shows a yellow wing-bar.   

It is hard to be sure which species this is.  Until recently, all the above were considered "races" of the same species.

Anyway, a rare bird and interesting record for people to discuss!

John