The small indentation on the upper cheek is called a Malar mound which can go on to develop into a malar festoon. I always believed these are due to local loss of fat, thinning skin, collapse of the medial fat pads due to aging and inadequate bone support due to zygomatic hypoplasia.
I was wrong. I have this problem and thought it was due permanent loss of fat until I noticed some of my family members have this too and for them it can fluctuate from noticeable to non-existent.
This phenomenon is caused by edema: the accumulation of fluid in a small area between the infraorbital rims and malar eminence. Over time the persistent malar edema causes chronic distension and attenuation of the orbicularis oculi muscle which combined with lax skin results in a "hammock" of muscle and skin called a festoon that you will mainly see in people of advanced age.
One might not experience edema in large amounts however the zygomatic ligaments are very prone to it and a "dent" on the upper cheek is an early indicator that a person is dealing with initial stages of lymphatic imbalance.
I started looking into this after seeing reports of people online saying that the indentation disappeared once they started up on thyroid medication.
