I am sure LE would have looked at the POI's mom's boyfriend...but no reason to think it was him more than anyone else, probably, no indication he knew Kayleah. And if it comes down to RSO's, there were probably plenty of others close by.
I guess maybe we will find out at some point when LE first learned about this guy as far as having sexually assaulted Kayleah; that is when I feel he should have been taken into custody, regardless of whether she was missing or not. I don't buy that they would leave him out there, knowing he is a danger, just in case he might lead them to Kayleah. If they did that, they are putting community in danger, IMO.