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I believe the live search does not search the whole forum history. The text above the box changes depending on where you are. If you are looking at the list of topics, then live search looks only in the list of thread topics. If you are viewing a thread then it searches comments in that thread only (I think).
The livesearch comments tool on the top left hand corner is the culprit. It seems like it doesn't do anything.
Thanks for showing me the tab Trogdor. I honestly didn't notice it.