Here is the reply from the customer on replicating the scenario:
When I do a Purchase Invoice for example using a Invoice from DHL, I apply the freight charges to a Warehouse Receipt, so that the charges get applied to the correct inventory.
My problem is, when I need to investigate a invoice that I know was tied to a Warehouse Shipment, I should be able to see this from the Posted Purchase Invoice. Currently I cannot find anyway of doing this.
Therefore I now have to make comments on the Posted Purchase Invoice for future reference.