How can Google know my Bounce rate?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  David Alanakyan: Various SEO experts insist that Bounce rate is a ranking factor today. How can Google know my Bounce rate in case I am not using Analytics and user is not using Chrome?
  It's seriously surprising how these SEO guys ignore that problem in their articles. #SEO #Google #BounceRate
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  Federico Sasso: They may just use the term improperly, for lack of a better one. What they might be referring to is "back to SERP rate", or "short click" analysis.
  So, they probably don't really mean BR as measured by Analytics, but users' behavior on returning back to the SERP. Google officially admitted having experimented it in the past, but says it abandoned it as it was unreliable, nevertheless some independent experiments seem to demonstrate it still has (had?) some weight, even if the model analyzed by Google was more complex.
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  David Alanakyan: +Federico Sasso Yeah "back to SERP rate" would surprise me less, but still even in that case if user is NOT back to SERP you don't know if they are spending hours on the site or closed the window and went to sleep.
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  Ryan Cramer: The only scenario I could think of where Google would have no way of tracking visitor behavior on a non-GA website is where Safari users are only using Bing or other non-Google resources... in that scenario, if the visit is not able to be tracked, a bounce cannot be attributed either. However, going from a Google resourced website (whether search or a website with GA tracking) to a non-GA website, can provide some information to Google on visitor behavior associated with your website (as before GA), including a bounce when a visitor hits the back button - back to the referring page.
  I just wonder why anyone who cares about their Google SERPs would NOT have GA installed on their site.
  
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  ?ukasz Rogala: There is a big difference between bounce rate in Google Analytics and "bounce rate" which can Google calculate between your website and terms you are ranking for. The first one can be adjusted by editing tracking code.
  
  Considering that having a high BR in GA can fck your rankings is just stupid. Especially if you read tthat you should work on tracking code to better measure BR based on website type. :)