I just marked one of my repositories as a private one, so no one can see it, or may know it even exists.
But a difference between the way bitbucket handles private repositories, and the non existing ones, confirms the existence of that repository.
If repository does not exist, bitbucket throws a 404, but if it does exist, but is private, bitbucket redirects user to login page.
IMHO denying the existence of a private repository is a better approach than the existing one.
Thanks for your great service,