I'm guessing this a feature rather than a bug <img src="https://www.ubbcentral.com/boards/images/graemlins/laugh.gif" alt="" />
A member of my forums (6.7.0) discovered the Search tool does not find all the instances of the searched item - a search limited to a specific forum, it finds two instances, but one very long thread (250 or so posts) in that forum actually contains the searched item too - and it is not reported.. what am I missing? Or is it a bug?
Also, the search index will only index so many characters of a post. This is controlled in your CP, Primary Settings, Search Tab. Keep in mind, that increasing the number of characters indexed, that will increase your disk usage.