"It would be shortly after four." "Someone had been hiding here, sure enough." He held down the light, and the marks of muddy boots were very visible in the corner.

"I'm bound to say this bears out your theory, Mr.Barker.It looks as if the man got into the house after four when the curtains were drawn, and before six when the bridge was raised.

He slipped into this room, because it was the first that he saw.

There was no other place where he could hide, so he popped in behind this curtain.

That all seems clear enough.