Without reviewing the mammograms and ultrasounds, we cannot speculate on what this could be and how
concerning it is. Certainly, implants
can and do complicated imaging but it is curious that this "spot" was there on 3 mammograms and 1 ultrasound and then disappeared. Since it was present on all of these images before, it may be reasonable to repeat the ultrasound. If it still fails to show up, you can discuss with your doctor what additional testing may be appropriate.