DotNetNuke is fully opensource, all of the code is within the source download.

ModuleID is in the file you're looking at. If you look about line 300 you'll see it's definition. It's readonly as it's only designed to expose an internal value. It's been marked as invisible to the designer for that reason.

<Browsable(False), DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)> _
        Public ReadOnly Property ModuleId() As Integer
            Get
                If Not _moduleConfiguration Is Nothing Then
                    Return Convert.ToInt32(_moduleConfiguration.ModuleID)
                Else
                    Return Null.NullInteger
                End If
            End Get
        End Property