I can't find the original source, but the system for deciding whether to just show a check box or not was super complicated.Back around a year and a half ago when it was first released, some people on 4chan's technology board (/g/) were frustrated because they'd repeatedly be marked as suspicious (and never got the check box only) when posting on 4chan (which requires a captcha to be solved for each post). One user in particular was reverse engineering it and published a ton of super interesting stuff on Github (the levels of obfuscation were insane), but later got a job offer from Google (allegedly) in exchange for deleting it.