observation post vs whereabouts

observation post

noun
  • a lookout or vantage point 

whereabouts

noun
  • Location; where something is situated. 

  • Information about an elite athlete's future whereabouts, supplied to anti-doping authorities to facilitate random out-of-competition testing 

adv
  • In, at or near what location 

How often have the words observation post and whereabouts occurred in a corpus of books? (source: Google Ngram Viewer )