[mcbits]

It's not trivial to match any arbitrary profile with an offline identity, but it is possible to cluster pseudonymous profiles into "almost certainly the same individual" by patterns and peculiarities in how they use their devices. If the same patterns later show up for an identified user, they can be linked with high probability.

With the sites Google runs plus running their own JavaScript on a sizable fraction of other people's web pages, they can pick up a lot of patterns, many of which would be inaccessible to police and intelligence surveillance.

Some people have nervous habits like moving the mouse around, clicking/tapping on whitespace, scrolling up and down, etc. Some always/never use the scrollbar. Some always/never open links in new tabs. Some tend to put the adjective before/after the noun in their searches. Some will rapidly open up the first 5 search results in new tabs. Some always disable instant search, and some of those change their settings to 20 or 50 or 100 results. Some use search features like the calculator, searching for "weather", stock symbols, etc, and others never do.