What's the first sentence a thief can't guard against?

This sentence means that even if we take all possible preventive measures, it is still difficult to completely prevent thieves at home. This sentence is often used to describe that even if we take various preventive measures, we still suffer from internal threats or dangers. Remind people to be alert to the potential risks within the family, whether it is thieves, fraud, or distrust and betrayal among family members. This sentence can also be extended to people who should always be vigilant when dealing with affairs. Some threats come from the most unpredictable places.