According to Ronald, he knew something was wrong when he got home because Misty was up and everyone is usually sleeping when he gets home.
I also agree, Misty would have been better off to pretend she was sleeping and let dad discover Haleigh missing, better for her if she is involved somehow and even better for both of them if they are both involved...why the convoluted story about the beds and the door and all that? Why not just say dad got home, front door was partially open, he ran to check on kids and GF and found Haleigh gone? Why the big deal about the beds, the backdoor, the cinderblock and all that?
This is what I am thinking...Misty often slips out at night to visit brother or friends while dad is at work. She goes out the backdoor...someone knows this, they have seen her do it before. So on that night, Misty puts the kids to bed at 8pm, she checks on them at 10pm and they are sound asleep, so she goes out the backdoor, leaving the kitchen light on. For some reason she props the screen door open with a cinderblock. She goes to another home nearby, loses track of time and sees dad's headlights coming up the road. She panics knowing he will be mad she is out, she runs back home, goes in the backdoor, dad is pulling up, she doesn't have time to turn off kitchen light, besides dad probably saw it was on, she goes to the bedroom and discovers Haleigh gone, now she is really panicked and she can't pretend she is asleep so she comes up with the bathroom story and meets Ron at the door.