No references attached, but still loading external files

Hi Everyone

To create our 3D models (e.g. Struct_Model.dwg) we reference in an overall model that contains the mechanical and electrical equipment.  When we are finished modelling, the overall model reference gets detached.

Now when I open Struct_Model.dgn there is nothing in the reference dialog box.  However, if I do a save-as dwg, in the progress dialog box it says 'Loading Mech_Model.dgn' and it iterates through all the files that used to be attached.  This makes it very slow.  

When I open the file using the -o switch in the command it doesn't happen.

Any ideas why the link is persisting and how to get rid of it?

Thanks