kleptēs

A thief works in the dark. The word points to someone who takes what is not theirs quietly and unseen, slipping in when no one is watching, as opposed to a bandit who overpowers people in the open (λῃστής). That is why a thief is so often paired in the New Testament with night, locked doors, and the householder who never saw it coming.

The word is used two ways, and both grow straight out of that one picture:

• Plainly, of real thieves: the person who breaks in to steal treasure, the false shepherd who comes only to take, Judas helping himself to the common purse.
• As a comparison, of something that arrives without warning: the day of the Lord, and the returning Christ, come "like a thief in the night." Here the point is not stealing but timing. No one is told the hour, so the wise stay ready.

Even in that second use the word keeps its plain meaning. It is precisely because a thief gives no notice that he became the standard image for a sudden, unannounced arrival.