He crept back quickly and silently out of the mouth of the passage, then he made a noise with his feet, turned again into the passage, and walked loudly towards his door.

Even so he was only just in time.

Had he waited a moment longer, he would have been detected.

For even as he turned the corner there was already a vertical line of silver on the passage wall; the door had been already opened.

But as his footsteps sounded on the boards, that line disappeared.
He walked slowly, giving his spy time to replace the letter, time to hide.