There are lot of cases, where the message source is not the channel or the BO layer in SRM. In this case, put your external break-point to the function BBP_PD_MSG_ADD. 

This helps you to identify the right source code part and logic adding this message, based on some criteria. This function adds an individual message to a collection in PD function layer. This message collection is passed layers above, but there you would have difficulties to find the origin of the message.

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