secret vs such-and-such

secret

noun
  • Private seclusion. 

  • A piece of knowledge that is hidden and intended to be kept hidden. 

  • The key or principle by which something is made clear; the knack. 

  • A form of steel skullcap. 

  • Something not understood or known. 

  • Any prayer spoken inaudibly and not aloud; especially, one of the prayers in the Tridentine Mass, immediately following the "orate, fratres", said inaudibly by the celebrant. 

adj
  • Being or kept hidden. 

verb
  • To make or keep secret. 

  • To hide secretly. 

such-and-such

noun
  • A placeholder or generic thing. 

det
  • indefinite; some or other, not stated 

How often have the words secret and such-and-such occurred in a corpus of books? (source: Google Ngram Viewer )