slip away vs slope off

slip away

verb
  • To leave a place, or a gathering, without being noticed. 

  • To die peacefully. 

  • To pass quickly, almost unnoticed. 

  • To disappear; to elude one; to recede; to dwindle. 

slope off

verb
  • To depart quietly, without being noticed. 

How often have the words slip away and slope off occurred in a corpus of books? (source: Google Ngram Viewer )