Note that Google's version of this was determined to be checking whether you had a 9-day-old tracking cookie.
The researcher who discovered this was able to generate 60,000 "I am not a bot" cookies per day, and use them up about 15 times each in a bot before it started getting captchas.
That's probably what it was. So they accessed some page over and over, pretending to not have the cookie yet, got a bunch of cookies, and 9 days later, used them to bypass captchas.
