IMO they would need to have a witness that saw the door opened, or video footage or this is a detail MT spilled in her 3 hour talk - all speculation of course because we simply don't know yet.
I don't think police should rely on the statement from one of the suspects, so that suggests that there is video of the door being left open. However, if there is video of the door being open there should also be video of all activities at the house that morning, and that does not seem to be the case.
Would police accept the word of one of the suspects?