Since I realized that probably not a lot of people know this: Google does network-level (ie. TCP/IP-level) fingerprinting to detect automated requests, in addition to HTTP fingerprinting and more 'traditional' methods such as cookies and IP tracking.

I only know with *certainty* that this is used for scraping prevention on the search engine, but I would not be surprised if this were used for user tracking as well, given Google's history.