What if the angel he had surprised had only come to appear to grannie in her sleep?
Why not?
There were such stories in the Bible, and grannie was certainly as good as some of the people in the Bible that saw angels--Sarah, for instance.

And if the angels came to see grannie, why should they not have some care over his father as well?
It might be--who could tell?
It is perhaps necessary to explain Robert's vision.

The angel was the owner of the boxes he had seen at The Bear's Head.

Looking around her room before going to bed, she had seen a trap in the floor near the wall, and raising it, had discovered a few steps of a stair leading down to a door.

Curiosity naturally led her to examine it.