The most common place for the brain to bleed after head trauma sits right against the temporal bone, which is one of the thinnest bones in the skull.
That is exactly why it gets missed. On a fast read, a small collection of blood blends into the shadow the bone already casts. The eye slides past it and calls the study normal.
The finding does not hide because it is faint. It hides because the reader did not slow down at the one spot that deserved the most attention.
In neuroimaging, knowing where trouble likes to sit is half the read. The other half is refusing to rush past it.