You have a reference to a file wich no longer exists.
How to create it for fun..: 
-In session createn a assembly and two parts as in example(asm0001.prt, prt0001.prt, tmp.prt). Do not save anything yet.
-First insert tmp.prt into the assembly then replace it with prt0001.prt.
-While replacing you should have checked "Remember These Components"
- Now in Refrence Viewer you'll see a trash can next to the tmp.prt. If you click on it you will break the link, but do not do it since we want the gost. Close the RefViewer 
- Now erase tmp.prt from session. Now in RefViewer you will see a X symbol in front of tmp.prt which means that the part is not open in the session. Still do not remove it coz we want to bust a ghost 
- Now save the assembly. Now you will see that a gost part will is created. 
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If you don't want the ghost, then you need to do the opposite. Never remember unused components, or delete the links in RefrenceViewer if you already have.