The teaching point is: malignant lesions do not have to grow to arouse suspicion. Increasing density in a lesion is also a red flag and deserves attention. Just because a lesion does not enlarge do not assume all is good. This case illustrates the point well.
Use a hanging protocol that allows you to compare density in the same image. Then once you are comfortable with this, change the protocol very couple of months so you "refresh/reboot" your observation skills. If you stay with one hanging protocol for years your eyes get lazy. Try it; it works.