They could use a little more explanation. Things most forum "searchers" are not aware of are:
Some threads are archived and may not be able to be updated.
Also, the box that asks whether you want a fast search, slow, crawler, etc. can greatly affect whether your search gets any hits.
It looks for the number of hits on each of your search terms until it hits the number specified. For example, a normal search says 1000 hits so 1000 hits will be returned. However, if all 1000 hits contain only 1 of 2 search terms (joined with an and), your answer set will be nothing!
This means the order and number of search terms -- in addition to the type of search performed can directly affect what you find!
Forum users need to know how to use the search function better. Part of that learning should involve better instructions on the search page. One could argue that better visibility could also exist, but I'm not getting on that bandwagon. I found it VERY easily.
BTW:
The video (HTML) link above about the search function is quite humerous.