If I run 'Detect Scenes' on the file in the Compact Library then the function works, for that scene.   It doesn't work on any other clip.

Update:-   OK, behaviour is erratic, and misleading.  'Find In Library' from timeline context menu actually creates the 'All Media' tab but does not scroll to the highlighted scene (or file).   Therefore, with the massive size of my Library I need to manually scroll to find the highlighted scene, which isn't very useful.