ACK showing up as Orphaned Message

Could someone explain how and why a HL7 ACK be showing up as a Orphaned message when I run the following SQL...
 

SELECT HL7.ID,HL7.DocType,HL7.Envelope,HL7.Identifier,HL7.MessageTypeCategory,HL7.Name,HL7.OriginalDocId,HL7.ParentId, HL7.TimeCreated
FROM EnsLib_HL7.Message HL7
LEFT JOIN Ens.MessageHeader hdr
ON HL7.Id=hdr.MessageBodyId
WHERE hdr.MessageBodyId IS NULL

I am trying to find the problem code that is causing the Orphaned messages, and an ACK showing up seems kind of Odd. While we do have Archive IO/Trace on, and Index NOT OK's set why would they show up as Orphaned messages?