Well, I've played around with it a bit, and here's what I think the issue is.
If you search the entire board for:
Paris
Using the default Advanced Search page with the default options, the following post should appear as a search result:
http://forums.footballguys.com/forum/index...opic=163570&hl=
However, it does not appear in the list. But when you do the same search from the default Simple Search page with the default options, the post above does appear as a search result as it should.
One thing I have noticed is in the Advanced Search for Paris, the following text appears next to the page counter at the top of the screen:
(Search results limited to first 1000)
However, when you run the same search through Simple Search, the result info that appears at the top of the page is:
Results 0 - 25 of about 1990.
So I figure that what is happening is that the result recordset is getting truncated in Advanced Search and the missing posts are part of the group that got truncated to bring the result set down to 1,000 posts. Even though the missing post was created yesterday, depending on the parameters of the query, it could be getting truncated -- there's no guarantee that the Advanced Search returns the most recent 1,000 matching posts, even though that would be the way I would do it.
The Simple Search screen apparently is not subject to the same 1,000 message cap as advanced search, though, so it can return everything including the word "Paris".
This would also explain why the more specific searches work while the more general ones do not -- for example, "+Paris +Simpson" is able to return the missing thread in question because the number of results for that searches is well below 1,000 posts, so you get the whole recordset back.
So I have put in a support ticket/bug report with Invision to see what they have to say about this. In the meantime, though, you will probably want to use the Simple Search to do your searching for popular terms that will probably return more than 1,000 results.
(I would re-enable Simple Search as the primary search engine but I seem to recall a great disturbance in the Force the last time I switched the default engine out.)