When you are in a course and you type a term in the search box and press "search forums" then Moodle only searches in the forums in that course. Is that an answer to your first question?
I am not 100% on an answer for your second question, although I cannot imagine that it would be otherwise.
Normally you get a decent/normal amount of results when you do a search. If you search in the Using Moodle course you often get lots of results and they are not always the ones you want to see.
One problem is the sheer size of the Using Moodle course. I am estimating between 2000-2500 messages every month.
General help
Moodle's Search tool
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