R.H.
I'm sure it's nothing to worry about. Although I know from personal experience, these sort of things can be frightening. It seems like when it comes to their heads, you wonder what's supposed to be there and what's not.
At my son's 4 month checkup, his pediatrician asked if he slept with his head to the left side more than the right side because he had sort of a flat spot above his ear. From the time he was born, I always switched how I laid him in his crib each time so that he would rotate his head to both sides equally. She then sent us to get a CT scan. She was worried that he had craniosynosis, which is a condition in which one or more of the sutures in the skull fuses prematurely. If that happens, the brain has no room to grow and expand. Surgery is needed to take the skull apart and piece it together to that of a 3 year olds. I was a nervous wreck until we had the CT scan done. We got the results back from his pediatrician and a pediatric neurosurgeon a few days later. Come to find out, he actually had an EXTRA suture on that side. They both told us how incredibly rare that is. We were so relieved!
My son does have something like what you describe, but it appears to have a vein underneath it. It indents in too. I was concerned about that too, but noticed other babies that have the same thing. His hair covers it up, so I don't even notice it now.
For your peace of mind, be sure to mention it to his doctor at his check-up. Please let us know what you find out.
Best wishes,
R.