How are my sites getting indexed?

I dont understand, I have 2 domains with the same name but different tld's still being developed.

I have no inbound links at all. One of them got indexed, but I didnt bother looking in to it as I thought maybe I did do something quickly to get the first few backlinks.

Now the domain has dropped off of Google's first 1k results and I cant check any further. Ok fine, it had nothing but a logo and some H tags so I guess I cant complain, although really annoying as it was on page 3.

Then at the same time, the other domain has been indexed (it wasnt before) and on page 6 for the same keywords. I knew I didnt do anything for this because it was going to just forward onto the other (main) domain.

Checking backlinks neither domain has ANY at all... so how have they been indexed?

I think google check newly registered domains, and that is the reason your sites get indexed.. It happens all the time..

If you dont want google to index your sites, block it with robots.txt