He did, apparently, enter a home seemingly to look for the children and then left (Blair's, I think).
Emily and her family lived in a pretty small space. I would think that if this guy came banging on the door, all of them would have come to see what was happening.
Also, Emily wasn't like the other children. She was a young adult, really, apparently with a big, spunky, outspoken personality. (The mother of her boyfriend spoke about her at length in one of the podcasts I listened to. She seems to have known Emily (and Lisa McCully) well.
I think Emily would have been well known to the killer because of her father's relationship with him, and because of her interests. She would have been more likely than the other children to have interacted with him because she was older, and because she was learning carpentry and welding. Additionally, there seems to have been some knowledge among friends regarding negative aspects of his relationship with his common-law. Emily sounds like the kind of young woman who could have had strong opinions about such things, if she was aware of those problems.
Edit to add: I think this was the podcast:
Dark Poutine – True Crime & Dark History – A podcast about notorious Canadian crimes, dark history and other creepy topics as told by real-live Canadians