This bird is generally brown in colour, with a poorly marked head pattern - key features of a female kestrel. Other falcons in HK (hobby, peregrine, amur falcon) are typically grey above with obvious black face markings. Hobby would also have a red undertail in adult plumage (which all should be at this time of year). Structure is also useful when you are familiar with the species - kestrel can look very long-tailed at times.
I think I saw this bird the other day - is it the individual along the Mai Po Access Road. It seems exceptionally worn and very thin, giving it a 'jizz' that doesn't initially seem very kestrel-like.
