Do SEOs index pages that are not linked to?

Question

Search Engine Optimization

Hello!
Does Googleyandex index pages that are not closed in robots.txt, but to which there are no visible PS links? These can be links that appear on the site only for registered users (PS view sites as Guests → they simply do not have these links). These pages themselves may exist for non-registered users and be available by direct access, but links from the site are only for registered users.
The indexing mechanism, it seems to me, can be based on the history of visiting site visitors (especially Chrome users for the Good Corporation or Yandex Browser for the domestic one). Here the robot looks, that a person visits pages that he himself did not get to, following all the links of the site, and goes to it.
Sincerely,
Alexander.

Answer 1

Yes, a hundred times I came across indexed admin panels, and indexed along with the mail. The reason is the spy functions of the Google browser.
For example, there is a translation function. I clicked "translate page" in the admin panel and that's it, the data was leaked

Answer 2

According to the conspiracy, it is possible that he goes where he should not.
But most likely the users themselves use these links somewhere and the robot still finds them.
Here the question is different:
how these pages are implemented for users who have not passed authorization - this is either an error page or a redirect to the authorization page.
With an error (404) or redirect (301), the page will not be indexed.
Well, unless they use Google / Yandex and someone else's password or, God forbid, register themselves! :)