How do search engines know what users are doing on a website?

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

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  Devin Peterson: How do search engines know what users are doing on a website (to measure User Engagement) if there isn't analytics tracking installed or if you're not using their associated browser or something like that...?
  
  Or do they not know and can only measure things like if the user returns to the search page?
  
  If they can't measure these metrics without a tracking code installed on the site, then aren't they missing out on a lot of valuable user engagement metrics like click depth, return rate, recency, etc... Even tracking interactions like "add to cart" or "register" would be very valuable metrics to gauge the usefulness of a site but I don't see how the search engines can know all those details unless you have something like Google Analytics installed, but then only Google sees it of course and not everyone uses that, so would it even be fair if Google DID use those metrics?
  
  Just need clarification.
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  Jeremy Estes: They cant see that stuff without tracking installed.
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  Devin Peterson: +Jeremy Estes ; So when they CAN see it, do they use it? Can they see it through their browser data?
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  Nick Fitzgerald: +Devin Peterson From the reading I have done throughout the years , some SEOs think Chrome was developed specifically to help them track user engagement outside of analytics. Cutts declined it in August, but in my experience they are definitely measuring engagement and it definitely helps. If they only used analytics data from people who have it installed, that would be a very clear advantage. My personal belief is that they use some Chrome data to work in conjunction with their algos to verify certain factors on (while using analytics data across the web to establish benchmarks), but that they do not collect massive data from Chrome itself. (Even though Bing has admitted to doing it). This is a great question that you pose, and one that it would be awesome if someone answered here. 
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  eBizWebpages: I think the only action they would admit to using to determine engagement is a bounce right back to the search results. That I know for a fact they measure. If Google was using Analytics to determine engagement and then basing serps on that then I think somebody would have evidence of that by now and alot less people would be using Analytics as a result.
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  Devin Peterson: So I came up with some results. I found 2 sources discussing third party web tracking abilities that can be so precise to measure key strokes and mouse clicks on virtually any site. Why would one of the worlds biggest companies not be using this, especially if they could get away with it? If you're interested (+Nick Fitzgerald ;, +eBizWebpages ;) , the 2 articles are ;https://cyberlaw.stanford.edu/files/publication/files/trackingsurvey12.pdf and ;http://www.newfangled.com/unlimited_vs_limited_web_tracking.
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  eBizWebpages: I don't think they could get away with it. Yes, they could do it. But there's too many people paying VERY close attention. The average user will tolerate alot. Until they find out that using Google's peripheral services is actually hurting them. Then they would turn on Google. If think Google knows that and I think they know they're already walking a fine line in the loyalty department. They've angered many with the changes over the last year and a half. All it would take is a proven revelation that analytics data is a factor in the algorithm and that would be the last straw. Open revolt would ensue.
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  Nick Fitzgerald: +Devin Peterson that's a great point. There is certainly a large contingent that thinks they do use that technology, especially since they have started to demonstrate less than ethical behavior in the last couple years. The fact remains that nobody knows, but that all we can do is look at correlative data and studies. Unfortunately, that makes it a matter of opinion until the truth comes out.
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  W.E. Jonk: With regard to browser tracking: Chrome does track some things that may be used by Google's search engine. ;
  
  https://www.google.com/intl/en/chrome/browser/privacy/
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  Devin Peterson: Excellent Find +W.E. Jonk ;. I love how they make a point that other browsers do it too.
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  Ashish Ahuja: they have various ways to check user engagement indirectly also, mostly related to brand engagement like click thru rate, bounce rate, brand name searches, co-citation, mentions, dns usage (they provide free dns), gmail (they do spider your emails on gmail), chrome etc. etc. They are also thinking on providing free internet just to get the data. Their aim is to be at the source of all entry points to internet just to track user engagement.
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  Nick Fitzgerald: +W.E. Jonk Very nice. Confirms what I suspected. I'm beating myself up for not searching for this first. ;)
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  W.E. Jonk: As far as I know G doesn't use Chrome for "ranking". However G does use it to personalize/localize searches. G also uses it for auto-complete and to check whether or not a site is down:
  
  If you navigate to a URL that does not exist, Chrome may send the URL to Google so we can help you find the URL you were looking for. We may also use this information in an aggregated way to help other web users - e.g. to let them know that the site may be down.
  
  And probably more things...
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  Devin Peterson: I think I've decided I'm going to assume that they do use some sort of advanced 3rd party tracking for UE to consider in their ranking function. If for no other reason, simply because they can, even though +eBizWebpages ; thinks it would lead to a revolution if they got caught ;)
  
  I don't think these metrics would be overly influential. But here's the kicker, doesn't Google have all the reason in the world to try to keep this concept from webmasters? Every time a new way to rank websites appears then people find a way to game it. If no one knew about PageRank it would probably cut down on a lot of spam. So if people don't know about "UE-Rank" then no one will try to deceive it. Nevermind the ethical backlash.
  
  But I could be wrong.
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  W.E. Jonk: +Devin Peterson ;PageRank is query independent. That is, whatever the query is PageRank is a ranking factor. The information send by browsers (cookies, IP address, UA [1) are query dependent. They are implicit added to the query.
  
  For example, if you are in Paris and you searching for the Eiffel tower on a mobile phone, it makes sense to give you directions. However if you are in London on a desktop it makes more sense to serve the knowledge panel.
  
  If you don't provide a better UE to mobile users, in comparison with other sites, G might rank you lower if the UA sends out a mobile one. ;
  
  So UE does matter for search engines (imo) but they are more query (implicit) dependent. Makes sense...?
  
  [1] To me Chrome does state that send more then other browsers.