They also changed it so that if you've seemed human in the past, they're able to determine if you're probabilistically a human now.

This data is a few years old but I imagine it's the same based on my experience.

They're using your cookie + IP + your account data to determine if you're probably a human.

A LOT of reCAPTCHA sites never prompt you. You only know if it's there because you're on Tor or something.