The potential confusion here is that Brownish-flanked Bush Warbler can be yellow on the underparts in first-winter plumage. This is the reason that Yellowish-bellied was formerly on the HK list (it has now been removed, so any claims of YbBW would be a HK first!)
I don't have much experience of YbBW except a few singing birds on the breeding grounds (when, as Ho-fai mentions, the song is very distinctive). Looking at pictures on the OBC website, I think it seems that YbBW has a dark eye-stripe, pale supercilium and short tail - in these respects, the Po Toi bird seems closer to Brownish-flanked. The Po Toi bird also has brownish flanks (which, based on name, seems like it may be a useful feature). Unlike Ho-fai, I don't think the description of the call rules out the alarm call of BfBW, but I don't know the calls of YbBW.
On balance, I think it is most likely that the Po Toi bird is a 1st winter BfBW, but I would welcome any additional comment from anyone with good experience of both species.
