You don’t even have to be logged in, or even go to the same website. There are a number of companies that offer free analytics tools to websites. A completely innocent website that just wants to know how many people have visited their page might use a free people counting widget from Google, Facebook, or wherever. Now every time that webpage gets loaded by a browser so does that widget. The widget itself doesn’t even have to report back to the original creators. If it includes something hosted by the original creators, then they can track the destination IP and browser tags.