What is the expiation for consensual intercourse during the daytime in Ramadan, under the assumption that the time for Maghrib has begun?

If intercourse occurs after the adhan, there is no sin in it and no expiation is required, even if it later becomes clear that the muezzin was mistaken and the sun had not yet set. Only a make-up (qada') is sufficient because the spouses believed that night had begun, and Allah has forgiven error and forgetfulness. This is the opinion of the majority of scholars. However, if the error is not discovered, then neither make-up nor expiation is required.