But 5A does have a "back door".
You are suggesting an intruder entered through the wooden door with a key.
If an intruder entered through the wooden door, why would he then from inside open a window which is next to the front door?
If he heard one of the parents approaching the front door, that window would be a silly escape route as he would climb out right in front of them.
I agree that 5A does have a back door, two if we include the parents patio doors.
But the fact is that historically, the parents have always used that back door (patio door) to enter the apartment. If the abductor was in the the childrens room he had to either scramble out of the window (and unless he were tiny it would be a scramble) .... or run across the sitting room to the front door ... and be seen
However if the parents changed their normal route, and were coming via the front door, he would be able to see them coming well in advance via that open window with raised shutters.
Altho they would be unlikely to see him, because they would be looking in the direction they were walking ... and the intruder would be out of their periferel vision ... the converse is not true.
The parents would have been illuminated by the street lights and he would be able to see their body shapes even thru the lower twigs of the trees, walking along the footpath ... and that would have given him plenty of toime to get away thru the patio doors.