I am experienced in Metal Detector Validation with a number of companies who package food for Coles. Metal Detector Validation requires that the production line Metal Detector is tested for correct operation. This is done by placement of certified metal test samples (eg 1.5mm diameter ferrous metal ball encased in a plastic housing) with the the product through the Metal Detector.
Coles requirement has always been to place the metal test sample under the product when travelling through the Metal Detector, although I have heard recently that the sample is now to be placed in the centre of the product. I have always thought that either Coles do not know what they are doing, or it is their intention to deceive. Guaranteed detection of the metal sample anywhere in the product must be achieved by placement of the metal sample in the worst case position for detection. This is the position in or on the product which is closest to the Metal Detector aperture centre. Conversely the most sensitive part of the Metal Detector is at the perimeter of the aperture and this is often under the product. So, detection of the test sample placed under the product or even in the centre of the product does not necessarily guarantee detection any where else in the product.
Do it right Coles!!!