This isn't a plothole, it doesn't even have any significant in the story. This should be called "filming error".
You may need to have some hair shaved in the anaesthetic room just before the operation. This is only done if it is absolutely necessary. Not all "surgeons remove hair, particularly for children. But having the area where the surgeon makes the cut (incision) free of hair may lower the risk of wound infection later. You may only have part of your head shaved, or the whole of your head. This will depend on where your brain tumour is." http://www.cancerresearchuk.org/abo...treatment/surgery/having-brain-tumour-surgery If it is partially shaved it can be concealed.