5th metatarsal fracture. 2 weeks later there was evidence of callus formation. now line is gone and no evidence it ever existed another week later?

Hard to say: It is unlikely things could have changed so quickly. It is probably more related to the techniques in which the serial X-rays were taken. Sometimes variations in the angle at which an X-ray was shot or in the degree of penetration if the film can make the findings different. But if you have little ir no pain and no fracture was seen on your last X-ray, I would not worry about it.