When a pack and go collects the files for shipment I sometimes get a part which is dependent upon a part/assembly which cannot be found.  I am assuming that this part has an external reference to the part or assembly.  However the part itself seems fine and resolves ok.  How can I find the problem?  All sketches seem to look OK.  No warnings, errors, or flags that I can see.  How can I determine the original reference connection so that I can fix or break it?