Wouldn't you think MY would have locked that inner door before going to bed? Of course Jason could have entered any door since he would have had keys. Isn't that the door where we have heard new testimony that MY's blood was on the doorknob? So the killer (IMO, Jason) left by that door and kept it unlocked. If MF had found that door locked she may have given up trying to get in to retrieve the ebay papers.
Actually, we are guilty of leaving our interior garage door unlocked often, knowing the exterior door is secure...I know, not smart.
I'm thinking it was just habit for MY
Still...it could have been locked and Jay used his key and failed to re-lock it in the panic after the botched plan?
...Or he left it unlocked so it would appear to be no forced entry.