[jckt]

I actually thought that the whole "mainland infiltrates HK police" conspiracy was just PRC-phobia (which is a real problem in HK when it becomes xenophobic, sadly) but there's actually been evidence of it (albeit not much). If you're interested you can find videos of a riot policeman saying 「退後一點」 to protesters (and other weird sentences), which is extremely awkward in Cantonese (instead Cantonese would be 「褪後啲」 or something, sounds totally different), especially if you're in an intense situation.