If the existing topic (destination) is set to unlisted, it does not get returned in the search results when searching by the topic title.
A work around is to search by the topic ID instead, this does return the destination topic, even if it is set as unlisted.
I have raised this with Discourse here.
Updated Sat Jul 15 2017 11:17
Whilst reported as a bug it has been updated to a feature request. It seems odd behavior but there is, for now, a work around.
Closing.