It wasn't like those observers could see Rean and Roan. They simply could feel the emergence of the Star Bodies, which they themselves had. They also used their own methods to break through the barrier that Roan did, but that's not all. Breaking the barrier was considered the easiest part of acquiring a Star Body. After so long, the top powers of the Realm of Gods had already found their own methods to go through it. Of course, they all keep those methods in secrecy.
The biggest problem in acquiring a Star Body...was that one would need to find the right star! And Roan did exactly that. Well, to be more specific, his own Starlight, which he and all the other living beings are born with, did. But that was also based on pure luck. Roan just so happened to get close enough for his own Starlight to feel the presence of its own compatible star.