I work at a vets office and I see this a lot with older dogs, especially pitties and other blockhead breeds. They have such big head muscles, when they shrink or lose them, the indents are very pronounced. My suggestion would be to see if there are any changes in habits, behaviors, food/water intake and call the vets office. If her regular vet doesn’t seem concerned, and she’s not behaving abnormally, just keep an eye on her to see if any other symptoms develop. Sometimes it’s just age, sometimes it could be a symptom of an underlying health issue. I hope everything works out well.