Google greeted me with this message while being behind a private proxy:
Our systems have detected unusual traffic from your computer network.
This page checks to see if it's really you sending the requests, and
not a robot.
I did my research and is seems that this is caused by automated searching, which I am not doing. I guessed that someone was doing automated search with the proxy I have (private proxy ehhh?). Now comes the part I don't understand and it differs from other questions: This only happens while using Safari and does not when using Chrome. Why is that? Did the automated searcher used Safari's User-Agent? I thought that google bans everyone using the same IP as the bot, which is unpleasant when using mobile bandwidth.
This is something that you will commonly face through TOR too.
Since a large number of people may be using the same server/exit-node as a proxy, Google receives a larger number of requests from a single machine. When it detects this, it asks you to solve a captcha to prove that you aren't a robot.
It doesn't matter if your using safari. Use chrome, and you will eventually receive the message too.