Based on the anecdotes of BK's previous behavior while living in PA, I wonder if once in Pullman, WA he continued his habit of chatting up female servers at restaurants and asking them where they lived. If he did, it's not out of the question that he would have tried this at Mad Greek where Xana and Maddie were servers. What if one of them mentioned the house, maybe not by its full address, but other identifying marks, e.g., "Oh yeah, I live with a bunch of friends in the big party house down by the fraternity houses"?
In their situation, unless they were getting super creepy vibes from BK, such a comment probably wouldn't have seemed especially risky. For one, they'd have been communicating that they live with a LOT of people, which typically would be a deterrent to anyone targeting the home for a nefarious purpose.
Another possibility is that he asked the question, got shot down, grew fixated on M and/or X, and managed to figure out where they lived through SM or other means.