Spotlight generally excludes OS X system folders (such as /etc and /bin) from its searches. But, if you know the name of the system folder you want to search, you can search in it anyway. For example, to search for a file in the /etc folder, select Go: Go to Folder in the Finder. Type /etc in the Go to field and click Go. With that folder now open in the Finder, type whatever it is you want to look for in the Finder window's Spotlight field. When the Search bar appears, select "etc" as the folder to search (instead of This Mac). If you select one of the found files and check its file path at the bottom of the search window, you'll see that the search results are indeed within one of those previously unsearchable folders.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090325091626579