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.
I am not saying it didnt happen exactly as being reported. But the high amount of cases recently of girls going missing at the hands of internet chatters, by choice or not, is a possibility that clealry needs to be examined.