This is not unusual. The imaging field of the CT scan is larger than the area of interest, in your case the kidney. The entire field is viewed and examined by the radiologist and often other things are found. Your doctors are to be commended for following up! Mammograms are often followed by ultra sound because they give the doctors more detail if there is an area of concern. Most lesions found in the breast are benign, as you have already experienced. Try not to worry until all the tests are completed and you know if there is something to worry about
! Good luck!