Then came a blessed interval of just a minute, in which neither sentry was in sight.
Altogether, there was a period of almost two minutes in which no eye would be fixed on Boris's window, unless the sentry chanced to turn and look back.
To make sure, Fred studied both men.

And not once did either of them look back or up.

Their attention did not seem to centre on the house at all.

It was as if their instructions were more to prevent a surprise attack from outside, or the coming of some spy, than to watch those who were already in the house.
Once he had made up his mind, Fred buried himself deeper in the shrubbery and risked using his pocket flashlight while he wrote a note to Boris, telling him what he had learned of the movements of the sentries.

He told Boris, also, not to draw up the rope at once, but to climb from his window to the flat roof, something easy enough to manage, and then to move along five paces.