This is a very interesting bird. It is presumably an adult (any pictures of the upperparts?) which makes the colour of throat and breast rather unusual - these areas are normally black to blackish on adult males. I wonder if this could be cumatalis the taxon which breeds in the Russian Far East and Korea (and which is not on the HK list). Most birds are the nominate subspecies which breeds in Japan. This is a potential split so worth keeping an eye on - note also rather similar distributions when compared to Pale-legged and Sakhalin Leaf Warblers
