How do they know this? Did the little sister follow the kidnapper and her sister around the houses as he attempted to exit? If not, how do they know his lack of knowledge of the house and what path he took to get her out?
Seems to me that if he broke in and found his way to the room he would just back track easily to exit.
Since he had to go off clean pathways to enter by the window, his shoes were probably dirty and left a tale of meanderings on the carpet/floors. It's a large, irregularly shaped house with 8 people living in it. Even if he'd been in there before, possibly as one of the workman doing the remodeling, he might still get lost, especially in the dark.