scout searches the events in the eventlogger, the same
data you can see from the conversation app. It than tries to find
the contact information for this messages.

From the logfile :
You can see, the search text is found in one message,
it found a user/sender name and
add the result to the result list.
I have no idea why it doesn't show any result.

nicolai

edit: Ah and of course, if you enable only the conversation search,
scout searches in the conversation text, it does not use the search
string to search for senders. So if you want to search for
messages from "mary" you will only find messages where "mary"
is part of the message text.