This case illustrates two points:
This is why MLO views are performed instead of ML views for mammography. You see so much more of the breast volume on the MLO view. A mammogram is a two-dimensional view of a three-dimensional structure; the same way a flat map is a relatively poor replica of the earth's surface (http://www.eoearth.org/article/Maps). MLO and CC show more tissue volume than orthogonal views. You know the lesion is low, deep in the breast because it is not on the ML view. It is at 7 o'clock
Check the profile of the pectoral muscle on the MLO view and you will find these lesions early in spite of the dense breast parenchyma of this young patient.
On biopsy, this lesion was clearly palpable with skin retraction and focal thickening. Biopsy demonstrated IDC. The patient never noticed an abnormality.