Sometimes a model element can end up in a System View Workset… it’s rare, but it can happen due to a type of file corruption. In Revit 2012 you can fix this thankfully-unusual occurrence without the help of Revit Support (warning: that you will need to re-enable worksharing and re-make any user-created worksets afterward):
Bonus: Some Revit customers use this tool prophylacticly and find that occasionally disabling/re-enabling worksharing helps clean their model and improve performance.