Just Another Dream?
Something I've picked up in Casey and Frodo without specifically looking for it (and would probably see in other characters if I did specifically look for it) is the way we can tell the moment they spot something even if they don't move on it instantly and, in this case, even though Frodo doesn't know what it is he's spotted: maybe it's just a bit of movement at the edge of his line of sight. Watch the eye shift between frames 4 and 5, and then the follow through to frame 11, which brings him a little more upright in bed. The shift from 11 to 12 shows the first awareness of what/who he's seeing. In the last few frames on this page, his eyes begin following Gandalf's movement into our line of vision - although Gandalf hasn't quite gotten there yet (that happens on the next page, because I cropped the frames differently).
I don't think Frodo has had a chance to react to the sight of Gandalf when we get to the bottom of this page. He's still in guarded and cautious mode, and knows Gandalf is dead. His appearance could be a trick, or a dream, or - most likely under the circumstances - a hallucination.