If you run the script Select * from SY00500 where BACHNUMB = 'nnguyen' does it also come up blank?
Moving on from that, what about the general ledger and the RM trial balance. We need to find how far this trx got through the system. This might be as easy as just creating the missing RM transaction.
Before I did that I would use SQL and search for the string that is the SOP Document Number. I would want to consider each table containing a field with that value.