Sorry to hear of your mother's experience. I'm surprised there wasn't a radiologic finding (that is very unusual), even as early as the first postoperative day there can be a problem. It is not an unheard of occurrence and sometimes it is due to a defective wire. The twisting used for sternum closing, the repetitive movement of the the thorax with respiration and upper body activity can cause migration of wire within the chest.
Hope your mother has recovered properly, and thanks for the question. Take care.