A thread at Search Engine Watch Forums named Google indexing parked domains from Registrar feed, shows a specific example of Google indexing and ranking a parked domain. In that thread members discuss possible methods that Google could have located the domain name. The notion of Google finding the page with the toolbar does not seem likely.
So what is it? One member replies that "Google must be using their new domain registrar status to add more domains to their database." What shocks me is this statement, "This is not only happening to parked domains, it's happening to domains that aren't registered and have never been registered."
I have noticed time and time again, that I would register a domain, make sure not to link to it anywhere, and all of a sudden Google would index it. Gary Price replied to the thread;
Accessing newly registered domains is easy, in fact I have an alert service (small fee) that every morning sends me lists of domains registered the previous day for any and all keywords I want. For any search engine to do the same and then add the domains to their crawl would be very simple.
That is my thought, and always has been. Logically, there must be some Web page feeding these newly registered domain names. And can there be a Web page that links to non existent domain names? I guess so, why not?