keep an eye open vs keep one's eyes open

keep an eye open

verb

  • To remain vigilant for someone or something. 

  • To remain vigilant for a possibly dangerous situation. 

keep one's eyes open

verb

  • be on the alert; watch carefully or vigilantly for something 

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