As mentioned previously, Search results are based on the scope of where you are when you initiate your search. So if you are in a wiki, the results will default to being scoped to that community's wiki content:
You can dynamically expand that to be the scope of the community you are in (which then returns content AND communities that meet the criteria):
That can be further expanded (again, dynamically) to be scoped to the entire site, which could include Communities members.
All of this can be done without needing to start your search again, providing a simple, easy user experience in looking for information that applies what you are looking for.