Kind of a silly workaround, but what you could do is this: whenever you start Inventor, start off by creating a new "dummy" Part file, and turn on the visibility of one of the Origin planes (this will "dirty" the file so it needs a save). Then just leave the dummy part open throughout your session. If you ever try to close Inventor, even if all of your other files are saved, Inventor will prompt you before closing to ask if you want to save the dummy Part file.
If you have Inventor 2018 or newer, I wrote a quick rule that will automate this for you. Just save it in one of your External Rule locations, then place it under the "New Document" and "After Open Document" triggers for "All Documents". After doing this, the first document you open or create during a new Inventor session will trigger the rule to create a dummy part file and set it to dirty, essentially causing Inventor to always prompt you before exiting.
Hope this helps you until a built-in option is implemented.
oProjectFile = ThisApplication.FileLocations.FileLocationsFile
oProjectPath = Left(oProjectFile, InStrRev(oProjectFile, "\"))
oDummyFileName = System.IO.Path.Combine(oProjectPath & "DirtyDocDummy.ipt")
Dim oDummyDoc As Inventor.PartDocument
Try
oDummyDoc = ThisApplication.Documents.ItemByName(oDummyFileName)
Catch
'Dummy doc not yet created. Create it.
'First get the current active document. This will be the document that was
'opened or created and triggered the Dummy Doc Create rule to run.
'We want to capture this document so we can re-activate it after creating the dummy doc.
Dim oActiveDoc As Inventor.Document = ThisApplication.ActiveDocument
'Now create the dummy doc.
oDummyDoc = ThisApplication.Documents.Add(DocumentTypeEnum.kPartDocumentObject, , True)
oDummyDoc.FullFileName = oDummyFileName
'Dummy doc created. Set it to dirty.
oDummyDoc.Dirty = True
'Restore the previously active document.
If Not oActiveDoc Is Nothing Then oActiveDoc.Activate
End Try
