Depends how thoroughly police were examining each room on that first recce. They would have been racing through, seeing who was alive, who dead, trying not to damage evidence, tread in spatter etc.
Sheila's body was the other side of the bed. Could that have been seen from the doorway? Would an officer have the viewed the scene and said "1 in here", particularly if they were using a mirror device?
One thing - there's a door connecting to other bedrooms on Neville's side of the bed. That was open, right?