Ok got a good one. Seems to be related to mod permissions...
When you go into a subforum (eg FI) there's a "search forum" option at the bottom left of the screen. As far as I'm aware that used to restrict queries to that particular forum.
I've just tried to use it and it shows results from within that forum, but also the forums for which I'm assigned mod status.. e.g. I'll go into NSW and look up "mongoose," and results come from NSW, Importing, and Car Audio. Tried it in a number of forums and the same thing is happening.