That gleam of silver was the moonlight shining between the bottom of one of the doors and the boards of the passage.

And that door was not the door of Wogan's room, but the room beside it.

Where his door stood, there might have been no door at all.
Yet the moon which shone through the windows of one room must needs also shine into the other, unless, indeed, the curtains were drawn.

But earlier in the evening Wogan had read a letter by the moonlight at his window; the curtains were not drawn.

There was, therefore, a rug, an obstruction of some sort against the bottom of the door.