If it is a standard Outbound IDOC, there is a good chance that it would be generated by the FM IDOC_OUTPUT_<message_type>. E.g. IDOC_OUTPUT_ORDERS.
There could be a custom program which might generating the standard as well as custom outbound IDOC using the FM MASTER_IDOC_DISTRIBUTE. If you put a break point at the FM and try to generate the IDOC, you would be able to trace back the program using the call stack.
Regards,
Naimesh Patel