We have discovered that you can create a vendor invoice and select a PO from the drop down and not bring any lines in. If you click update match status, it will show as passed even though there are no lines from the PO on the vendor invoice. Once it is posted, if you search vendor transactions the invoice shows the unrelated PO number beside it. If you go to that PO, the vendor invoice is not listed. We had submitted this as a bug and were informed that this was how it was designed and that submitting it here was the only way to get it fixed.