A schedule agreement should not be displayed in MD04 at all, you should only see the delivery schedules when you call the MD04 for the component material.
yes you may see the sales order number in the reservations created from the production order which has a direct link to the sales order in a MTO scenario.
the Pegged Requirements might be a possible way to trace it - had not considered to check from the other angle - at least as long the schedule is active in MD04. I am not sure if this is still possible for historic data.