Because of the speed involved, no will see the rapture happen, I assume. Perhaps, that is what the authors means by secret. (?)

1 Cor. 15:51-58

Behold, I tell you a mystery; we shall not all sleep, but we shall all be changed.

In a moment, in the twinkling of an eye, at the last trumpet; for the trumpet will sound, and the dead will be raised imperishable, and we shall be changed..........