Frank J. Wilstach, comp. A Dictionary of Similes. 1916.

Secret (Adjective)

Secret as rocks under sea.
—Thomas Lovell Beddoes

Secret as the grave.
—Lord Byron

Secret as the head of Nilus.
—William Congreve

Secret as the night.
—William Congreve

Secret as thought.
—Francis Fawkes

Secret as a confessor.
—John Gay

More secret than a nest of nightingales.
—John Keats

Secreter than the isle of Delos.
—Anonymous

Secret as the magnet-stone.
—George P. Lathrop

Secret as your midwife.
—Edward Sharpham

Secret as a coach-horse.
—Richard Brinsley Sheridan