Google: a new kind of Captcha December 2014
Google is introducing a new Captcha, instead of trying to read a distorted word and write it down (because robots have troubles reading distorted words) you will just have to click on one button. Google will analyze small cues that prove that you are not a robot (like the movement of the cursor before clicking on the button). Some of those cues will only happen when the mouse will hover the widget, as google is not in control of the entire document when used outside of google's domain.
someone on hackernews:
It's definitely not only relying on cursor movements. A simple $('iframe').contents().find('.recaptcha-checkbox-checkmark').click() proved that I'm not a robot, without me touching the mouse.
It seems to rely heavily on your cookies as well.