Consider two ideas. A thief may come and go secretly, without being detected at the moment. On the other hand, a thief may come openly, but suddenly, without warning. If the figure were never explained in the Bible, your guess would be as good as mine. In the other five, the idea is always lack of warning.
Secrecy is never an issue.
Be ready at all times. Thief great noise earth burned up. No Bible text hints that Jesus coming will be hidden from the eyes and understanding of the masses. When Jesus comes, there will be no secrecy and no second chances. Eternity will have arrived.