1. A detection method comprising: acquiring first identifiers of items to be transacted during retail checkout; acquiring images of the items to be transacted during retail checkout; comparing the acquired images with a database of item information to obtain presence or absence of second identifiers of items associated with the acquired images; and detecting differences between a set of said first identifiers and a set of said second identifiers by comparing presence or absence of the first identifiers with the presence or absence of the second identifiers, wherein the detecting is adapted to be performed identifier by identifier and between the sets as a whole.