The search engine for the site is a third-party add-on. The supported first party options were huge resource drains "back in the day". I could probably try them again now since we're on a better server. It's honestly just not something I've spent much time on since I'm perpetually waiting for the next big version of the forum software.
FYI, the built-in search box at the top of the page only searches the currently viewed thread or subsection. To search the whole forum you'd have to be on the main index or use the advanced search link.
