Of course, it's possible. But, what a terrible coincidence if the one time Allison forgot to lock the back door, she happened to receive an unwelcome visitor.
I'm not sure how the locks on the doors work. On both my back and front doors, I have two locks. The top one is a deadbolt. There is a button on the door handle that releases the lower lock. There have been times when I had company my guests have released the lower lock and I didn't realize until the next day when I tried locking the door from the outside. If not for the deadbolt lock, anyone could have walked into my house (supposing the screen door was unlocked or tampered with).
If the locks on Allison's doors are similar to mine, someone working in her home earlier that day or earlier in the week could have released the lock on either the front door or back door. Maybe she didn't lock the deadbolt until later, especially if it was the front door lock that was released and Allison was in another room when her BF left. I'm sure, though, that the BF would have made sure the door was locked when he left. And, I don't think LE has said anything about someone being caught on security cameras entering the front door after BF left. Maybe Allison routinely didn't lock her deadbolts until she went to bed at night. So, someone who was watching the house closely would know when BF left and would know what time she turned out the lights at night. JMO
