If the "No Test Cases" checkbox is enabled in the Find/Replace pane and the current document is a test case (determined by looking at the file name), then Find will return matches in the current document.
If "Search All Documents" is enabled, then Find should skip the current document and start looking for matches in the next document.
If "Search All Documents" is disabled, then Find should report that no matches can be found.
When I originally implemented this feature, I did it in a half-baked way that only worked for Find All.
Fixed as of revision 4882.