An older expression in pretribulational literature—and can still be used occasionally today—is secret coming or secret rapture. From my exposure to older pretrib and non-pretrib literature, it seemed to be used synonymous with the expressions signless coming or signless rapture, in contrast to an announced coming or announced rapture. Darby attempted to answer objections to his secret or signless coming (see The Collected Writings of J. N. Darby, edited by William Kelley, 177–92, vol. 11: Prophetic No. 4. London: G. Morrish, n.d. reprint, Oak Park, IL: Bible Truth Publishers, 1962).
In recent decades, the “secret” term seems to have expanded its semantic range to include the aftermath of the rapture, when the world will look around in bewilderment as they scratch their heads wondering where millions of Christians have vanished. This sense seemed to have originated, or at least was perpetuated, by pretrib novels and movies beginning in the 1970s.
The term today can still be found in popular pretribulational circles, while pretrib scholars tend to distance themselves from using that expression because of its sensational connotation.