Drake tried to hide the surprise and by the looks of it, he succeeded. She was not looking at him so it was pretty easy. But that made him think of something else. She was looking down, what if she does remember and was not telling him to keep him out of harm's reach?
That was a possibility too which he can't rule out.
Ahh, well, sooner or later he will find out. He had told others that she fell and hurt herself and nothing more. Darren was feeling guilty so he took advantage of that and let him be their guide for remaining days while he accompanied Shirley. When the doctor told him about her injuries his doubt about struggle was confirmed.