investigate vs keep one's ears open

investigate

verb
  • To inquire into or study in order to ascertain facts or information. 

  • To examine, look into, or scrutinize in order to discover something hidden or secret. 

  • To conduct an inquiry or examination. 

keep one's ears open

verb
  • to listen for important information. 

How often have the words investigate and keep one's ears open occurred in a corpus of books? (source: Google Ngram Viewer )