That'll work, but I'm pretty sure Google can only search the boards that are visible to unregistered guests.
They have probably registered and posted enough to search the board. The board is running pretty common software, google is very smart.
With a major big box retailer, I\we have done some testing to see how good. The travel, shoppping and news portion is unbelivable. With shopping I could put an test\fake item out there with a fake SKU at a local store in BFE and do a google search including the store number of local zipcode. The Item and price will usually show on google shopping. If the end site is quick enough, it will find it as you type into the box. The autofill servers many purposes, one it to find new stuff it has not logged. After you search the item (giving you a price check) the item will continue to be searched weekly for over a year.
(different project)I watch logs at times to see where the traffic comes from I will post something, and 3 people will look at and test something I wrote. Sure enough, I will get a few "users" from wierd areas (like Manilla) that look at or watch an entire 3 hour video, and download the transcprition. Do a little diggin on the IP, and it is a known web crawler for google or one of the thousands of companies it has purchased.
I would love to know the basics on how it works. Even people inside Google do not know the full potential of that search bar. Want to see your site clime in google rankings? Use some Google open source for a bit.