Misty may have taken out garbage through that door and thought she locked it, but didnt' have it closed all the way so the dead bolt slid but not into the door jamb.
The A/C guy may have used the door and didn't think about locking it.
HC2 may have opened the door while he was there.
The kids may have opened the door while they were there.
Maybe Ron's normal procedure is to check the lock before he leaves for work, but didn't that time.
Whatever the case, I don't think it matters, because I think the intruder came in through the front door and used the back for the exit. I also think the doors were propped open because the intruder was going to empty the house of valuables down the ramp to an awaiting vehicle.
Before anyone says there was nothing of real value let me recount a true story. We had a used 2 bedroom mobile home on our farm land for weekends. It was filled with old furniture that would have been sent to the Goodwill. Everything in it was used and old from sheets to the rabbit ear tv. One weekend we arrived to find it had been emptied of everything, including bath towels and matresses. One man's trash - another's treasure.
I often wonder who was so desperate that they needed all of our old mis-matched, used furnishings. Apparently, someone did. Funny thing, if I would have known someone was that depserate, I had a lot more stuff that I could have given to them to set up a house. But, I guess stealing is better than asking for help. Who knew?