I have found that when you use the search box within a sub-forum it will search only that forum. For site wide search you need to search on the main page.
There should be a grey "drop down" box (aside of the green button with the magnifying glass. You can use the drop down box to select where you want to search - Selections in a topic are This Topic, Forums, Members, Help Files, Blogs, Store, Pages, Gallery, and Calender. Selections in a sub-forum are This Forum, Forums, Members, Help Files, Blogs, Store, Pages, Gallery, and Calender. The search function is preset to search the topic if viewing a topic, search the sub-forum if you are in a sub-forum, and search all forums if you are on the main page.
Thanks Takao, you hit the nail there. The default is the section you are in, which is usually what people are concerned with. A switch to the entire forum is quite easy as Takao helpfully pointed out.
