Please check Tcode: SXMB_ADM -> Integration engine configuration -> Change configuration :
You should have a line there under DELETION category called: Persist duration with subparameter SYNC / ASYNC
There you have the amount of time in days that a message is saved.
About the IDOC - try to look for it via payload by using Tcode WE09 in your ECC system.
regards,
Imanuel Rahamim.