You never know when someone might stab you in the back.
Literal
When throat-slit-in-sleep understood thing [copula-ではありません].
寝首を掻く 'to slit someone's throat while they sleep' — an idiom for betrayal or a surprise attack. いつ…かわかったものではない 'there's no telling when' expresses unpredictable danger. The passive 掻かれる makes the speaker the potential victim.