I noticed that Google's "I am not a robot" reCAPTCHA forces me to check correct images on my computer. I installed a virtual machine and tried there. Same thing. Used proxy. Same thing too. Then I used another computer in the same network (same public IP), but this time the reCAPTCHA doesn't force me to solve it. It just checks itself when I click it.
Very curious behaviour. I repeated the process a couple times with a few days in between, and some computers never need to solve reCAPTCHA, while others (including brand new virtual machines) behind a proxy need to. I even tried a new browser in a fresh new VM. I'm on a home network, not an enterprise network. I am confused about what triggered reCAPTCHA into thinking it needs to double check me even when using new virtual machine behind a proxy?
On computers where it isn't suspicious, I can delete all the cookies, history and caches, visit a website and reCAPTCHA just lets me go without any concerns. So it can't be solely based on my past activity. On the other hand, if I indeed solve the reCAPTCHA and register for an account on a website, the website is missing all the functionality for registered users.
Also, when I'm presented with a CAPTCHA, even on brand new VMs, the functionality of registered users is limited. Which leads to thinking that reCAPTCHA sends information of what it thinks about a specific user to the website owner. Is this a documented behaviour?