nonplus vs warning bell

nonplus

noun
  • A state of bewilderment or perplexity. 

verb
  • To bewilder or perplex (someone); to confound, to flummox. 

warning bell

noun
  • A problematic or alarming aspect. 

  • A bell rung to warn people of a danger. 

How often have the words nonplus and warning bell occurred in a corpus of books? (source: Google Ngram Viewer )