Hi Iñaki,
From what I saw, the specific table for documents (xxx18) is always empty, but you can always get the needed information by accessing the view B1_OinmWithBinTransfer, you'll have all the needed information and much more.
From this view, to go to OBTL you have to use the InvPlMessageID column (MessageID in OBTL) and from OBTL, BinAbs to have the corresponding OBIN object.
Query should not be to difficult...
Regards,
Eric