Subdural fluid can occur as a result of head trauma, external hydrocephalus, or as a treatment complication of CSF shunting and craniotomies. It is critical to differentiate simple hygromas. The most common cause of hygrona fluid-filled sacs that develop as a result of repeated trauma or pressure over a bony prominence. This would certainly be consistent with the caving in feeling you mentioned. In this case the answer as whether or not its a problem that needs specific treatment will come from your surgeon. In simple terms its a blister at the craniotomy site. Blood work would tell you if its infection. If its a shunting problem, you would know it. Merl can explain that one. The fix may be time.
That being said, the manifistations of injury and brain surgery are amazing and very subtle. My great grandaughter has spina bifida (repaired) and had a tethered cord as well as a hygrona. My wife and I were caring for her as her mother had massive complications from her SLE. Playing around I learned that Miles Davis music was soothing to her. I was fortunate enough to allowed into her surgery. Because in addition to the duraplasty (a type of craniotomy) she was having spinal cord surgery, she had to be semi conscious. There is a neurol specialist present that is tracking every nerve twich etc as the surgeon works so as to avoid any nerve damage. He is also in charge of patient soothing. I mentioned how Miles Davis Music soothed aurora so changed to music to smooth jazz. Immediately there was a change in Auroras brain waves not for the better. So here I am in the OR with arguably the top pediatric neurosurgical team in the world explaining that just because Miles Davis is a renowned Jazz musician, he plays with a 4th note blues conversion (very subtle but it is his unique sound). He changed to actual Miles Davis from the jazz play list and immediatly the 2 yearolds brain waves changed for the better.
All this to say each of our brains are unique and while we can have some commonality in experience, “Normal” is very individual and varies greatly. There is no formula and self research can drive you nuts. It takes a while to learn who “you” is. Its the same with your team. Putting all the little pieces together is truly a journey with lots of forks in the road. Don’t take a single piece of info and try to make something of it. It cannot be done.