Once again, in trying to help someone in an Access forum, I came up with the following procedure(s) to be able to identify currently open, but hidden forms.

Option 1 – Print the results to the immediate window

'---------------------------------------------------------------------------------------
' Procedure : ListHiddenOpenFrms
' Author    : Daniel Pineault, CARDA Consultants Inc.
' Website   : http://www.cardaconsultants.com
' Purpose   : Generate a list of current open, but hidden forms and print their names to
'             the immediate window.
' Copyright : The following may be altered and reused as you wish so long as the
'             copyright notice is left unchanged (including Author, Website and
'             Copyright).  It may not be sold/resold or reposted on other sites (links
'             back to this site are allowed).
'
' Usage:
' ~~~~~~
' Call ListHiddenOpenFrms
'
' Revision History:
' Rev       Date(yyyy/mm/dd)        Description
' **************************************************************************************
' 1         2014-Nov-19                 Initial Release
'---------------------------------------------------------------------------------------
Function ListHiddenOpenFrms()
On Error GoTo Error_Handler
    Dim DbF     As Form
    Dim DbO     As Object
    
    Set DbO = Application.Forms 'Collection of all the open forms
    
    For Each DbF In DbO    'Loop all the forms
        If DbF.Visible = False Then
            Debug.Print DbF.Name
        End If
    Next DbF

Error_Handler_Exit:
    On Error Resume Next
    Set DbF = Nothing
    Set DbO = Nothing
    Exit Function

Error_Handler:
    MsgBox "The following error has occurred." & vbCrLf & vbCrLf & _
            "Error Number: " & Err.Number & vbCrLf & _
            "Error Source: ListHiddenOpenFrms" & vbCrLf & _
            "Error Description: " & Err.Description, _
            vbCritical, "An Error has Occurred!"
    Resume Error_Handler_Exit
End Function

Option 2 – Return a delimited listing which can be used elsewhere

'---------------------------------------------------------------------------------------
' Procedure : ListHiddenOpenFrms
' Author    : Daniel Pineault, CARDA Consultants Inc.
' Website   : http://www.cardaconsultants.com
' Purpose   : Generate a list of current open, but hidden forms and return a delimited listing which 
'             can be used elsewhere.
' Copyright : The following may be altered and reused as you wish so long as the
'             copyright notice is left unchanged (including Author, Website and
'             Copyright).  It may not be sold/resold or reposted on other sites (links
'             back to this site are allowed).
'
' Usage:
' ~~~~~~
' Call ListHiddenOpenFrms
'
' Revision History:
' Rev       Date(yyyy/mm/dd)        Description
' **************************************************************************************
' 1         2014-Nov-19                 Initial Release
'---------------------------------------------------------------------------------------
Function ListHiddenOpenFrms()
On Error GoTo Error_Handler
    Dim DbF     As Form
    Dim DbO     As Object
    Dim Frms   As Variant
    
    Set DbO = Application.Forms 'Collection of all the open forms
    
    For Each DbF In DbO    'Loop all the forms
        If DbF.Visible = False Then
            Frms = Frms & ";" & DbF.Name
        End If
    Next DbF
    
    If Len(Frms) > 0 Then
        Frms = Right(Frms, Len(Frms) - 1)   'Truncate initial ;
    End If
 
    ListHiddenOpenFrms = Frms

Error_Handler_Exit:
    On Error Resume Next
    Set DbF = Nothing
    Set DbO = Nothing
    Exit Function

Error_Handler:
    MsgBox "The following error has occurred." & vbCrLf & vbCrLf & _
            "Error Number: " & Err.Number & vbCrLf & _
            "Error Source: ListHiddenOpenFrms" & vbCrLf & _
            "Error Description: " & Err.Description, _
            vbCritical, "An Error has Occurred!"
    Resume Error_Handler_Exit
End Function

Hopefully this can help someone else out.