Search boxes and similarly dependant items (like the "Questions that may already have your answer" div) should return answers from all sites.
Search box: If a casual user is searching for something, they shouldn't need to know which site to search. At the moment the process is that they go to the most likely site based on site title and search there. Not everybody knows the 90+ sites we have.
Questions that may...: A user could notice that the same question they are asking belongs to another site, or has been answered elsewhere and thus not create an off-topic question on the site.
This specifically refers to the searches on the single sites and it is a different request from this.