From https://support.google.com/websearch/answer/86640
You may see "Our systems have detected unusual traffic from your
computer network" if Google detects that a computer or phone on your
network may be sending automated traffic to Google.Automated traffic includes:
Sending searches from a robot, computer program, automated service, or
search scraper Using software that sends searches to Google to see how
a website or webpage ranks on Google
When this happens in Google Books, what are some possible criterions that Google Books or similar sites can use to detect that a compute may be sending automated traffic to their sites?
Best Answer
Google uses complex anomaly detection systems, which are getting popularised with the NoCaptcha ReCaptcha.
Everything started with a mere counting of accesses, of course. You can have fun looking for more Google stuff.