Excel VBA: Getting the workbook object of an add-in's referring workbook

I can create a code add-in by setting a workbook's .IsAddin property to True, and saving it in *xlam format.

I can then add this as a reference to any other workbook as such:

Sub Test()
    Dim wb As Workbook
    Set wb = Workbooks.Add
    wb.VBProject.References.AddFromFile "PathToAddIn.xlam"
End Sub

The add-in's code procedures will then be available to call from within the main wb workbook.

However, what if when running a procedure in the add-in, I want to get the workbook object of the workbook which references the add-in?

For example, something like this:

Sub ThisProcedureIsWithinTheAddIn()
    Debug.Print ThisWorkbook.Path ' Returns the path of this add-in (xlam) file
    Dim ReferringWorkbook As Workbook
    Set ReferringWorkbook = ThisWorkbook... ' How does one reference the caller/referrer?
End Sub

For the purpose of this question, assume that the code execution is in a completely separate thread, and the wb object (as originally created) no longer exists as a variable. In fact, in my application the wb has been created in an entirely separate New Excel.Application.

I realise I could go through each workbook in the application with For Each wb In Application.Workbooks, and find some relevant matching criterion. But that's a messy solution. Is there something more elegant?