Okay, with regard to SA, I know nothing about him, except what I have read here, and while I agree that things seem hinky with him, these are just my thoughts...
I do not necessarily find it strange that he drops by the house on a regular basis and at various times throughout the day. We do not know what his line of work is. He may have a job with variable hours and may drop in and out for lunch or just to hang out if he has some down time. I personally do not find it all that strange. I had a BF who used to do the same because my house was closer to where he would do most of his work. Maybe Billie's house is closer to where he generally works, maybe not. We just don't have enough info. But apparently Billie did not find it strange for him to drop in at various times. She assumed that he dropped over at lunch time, but maybe whatever he does for a living cause him to work later that day than normal and he took a late lunch or simply got off early. We just don't know.
I have a feeling that Billie was pretty lax with the kids, not necessarily in a bad way, but more of a small town, nothing will happen to my kids kind of way, as far as arrangements and when and where they are at various times. I don't find it all that strange that Hailey just told SA in passing that she was going ot her dad's and then to her friend's house and to have him tell her mom. Especially since it was Christmas break, the rules may have been even more relaxed. If it were during school time, maybe the rules about when and where the kids are is more strict.
I also don't find it strange that she was on the phone for a 1/2 hour outside in the backyard, if that is indeed the case. I recall spending tons of time on the phone talking to friends etc. Sometimes, if it was nice, I will go outside, not necessarily to get away from people, but just because I liked being outside.
Anyway, as I said, I do find issues with SA, but wanted to show a different perspective.
