RE: IFC S2R1 - If the Universe Is So Big and Old, Where are All the Aliens?
It would depend on the orbit of the ring world compared to us. When we are looking for earth like planets we use the same method we would to search for a Dyson Sphere. Unfortunately we can only see planets that are on an orbit that has them moving infront of the star relative to us.
However, any civilization capable of producing a ring world is going to be several times more capable of producing a complete dyson sphere. That's a lot of power they would be skipping out on.
Its possible there is something stopping the aliens from creating them. But considering how simple they seem to make, I can't see a super valid reason not to. Perhaps an interstellar civilization made it illegal, but its kind of like posessing nukes on earth. Anyone who can tell you not to create nukes, has nukes themselves.
If fusion is mastered, that takes a big motivation away. Though, it still doesn't solve the problem because why wouldn't you make use of all that wasted energy anyway?