He gave three sharp tugs, and then settled down to wait, with beating heart, for now the crucial test was coming.

The other sentry was about to appear.

If he noticed the thin string, by any chance, the whole scheme would be spoiled and Fred, in all probability, would be caught and treated as a spy.
The man came around the corner of the house, walking slowly, his head down.

As he neared the twine he stopped for just a moment and looked up.
Fred scarcely dared to breathe.

He knew what had happened.