I'd be interested in seeing if you could answer non-directed messages against a search stream instead of a friends stream.
To answer to your question:
* It will scan the tweets of people you are following too, as well as anyone @mention'ing you - respond if there's matches (so it doesn't have to be directed at you)
* Specifically, you said a search stream. I actually had that months ago, then much to my disappointment, after lots of sweat to get it working reliably, I found out if you're auto-responding based on a search, Twitter will not be happy: http://help.twitter.com/forums/26810/entries/76915 so I had to throw out that part ;(
I wonder if they would be open to dialogue about the issue though. For example, if someone is tweeting a specific question about a branding finding that with search and answering seems legitimate and helpful.
Obviously automated search stream based marketing sucks.