Imported assembly saved as part file

I'm an electro-mechanical designer, so I'm often (every day, essentially) importing component files from manufacturers that I need to use in my assemblies.  These are very often IGES or Step files that convert to assemblies with too many parts when they are imported.  I have been in the habit for over ten years now, of saving the assembly as a part, with "all components" selected, and then using that "dumb" multi-bodied part in my work.

Over the past couple of days I've encountered something new.  A Hoffman electrical enclosure came in, as expected, as an assembly with about 25 parts in it.  I did a save-as, saved it as a part file, and then made three configurations of the part (backplane only, no door, all on).  This is all standard stuff.  Somewhere along the line though, the part file started misbehaving, with the backpanel configuration working fine but the other two coming up with only the backpanel showing.  An alert said that the external components couldn't be located, which is fairly alarming.  I have always found the file to have no external links or references, which is how I like it.  I closed and reopened the part, pausing to check if external references were coming in with it and there were none.  Weirdly, after I'd found and opened the assembly (because I planned to replace the part-that-used-to-be-an-assembly with the original assembly) I found that all the missing bodies had come back.

So which is it?  Does the part file have references back to the assembly?  Or was that a glitch in the matrix?  (One that happened twice, since I deleted the part file and started over and saw it again later that same day).  I'm discouraged, to say the least.

Oh right, by the way, my customer is using SolidWorks 2013, SP5.0.  In case that matters at all...

Thanks,

Danielle

SolidworksAssemblies