"Since the launch of No CAPTCHA reCAPTCHA, millions of internet users have been able to attest they are human with just a single click. Now we're taking it a step further and making it invisible. Human users will be let through without seeing the "I'm not a robot" checkbox, while suspicious ones and bots still have to solve the challenges. We’re excited about this and hope you will give it a try!"
More here: https://www.google.com/recaptcha/intro/invisible.html