Christ is pictured as coming as a "thief." Does this mean He will return secretly—invisibly—in a clandestine manner? That is a fair question. But here is another question to ponder: When thieves or burglars rob a home, do they come invisibly? Of course not!
A thief, however, usually comes at night when the household is away or asleep. He catches them unaware, off their guard—he surprises them! One never knows when a thief is coming. If one did, one would call the police and be prepared for him. The big question a person would want answered if he already knew a thief was planning to rob his home would be when.
So it is with the coming of Christ. He will not come invisibly—any more than a thief comes invisibly. But He will come unexpectedly! No one will know exactly when He is going to come.
Jesus Himself explained: "Watch therefore: for ye know not what hour your Lord doth come. But know this, that if the goodman of the house had known in what watch the thief would come, he would have watched, and would not have suffered his house to be broken up. Therefore be ye also ready; for in such an hour as ye think not the Son of man cometh" (Matthew 24:42-44).
And so Jesus likened His coming to a "thief" in respect to the time of His coming and the element of surprise—not whether He would be seen (Revelation 1:7)!