(Cantonese analogy- figurative meaning) a soundless dog will bite someone to death
Definitions (粵典–words.hk)
語句
提醒人要提防啲唔聲唔聲、作風低調隱秘嘅人，唔知幾時可能會害你，就好似啲狗唔吠之後咬你一啖 an idiom saying that one should be careful of people who remain silent, one may not get any warning of a serious strike in advance; literally: a silent dog will bite you to death