I think the GNR officer may have been outside, as he says he didn't enter the apartment;
As regards the searches, he say that he did not enter the apartment, as it had already been searched, but instead remained outside. When questioned he replies that he remembers that the blinds of the window of the child's room were open, but inclined (the lower part, at about a hand's width). He does not remember if the window was open or what the state of the curtains was. He did not enter the room because he was told, he does not remember by whom, that the twins were sleeping.
http://www.mccannpjfiles.co.uk/PJ/ANTONIO_DUARTE.htm
The problem with staging is that the window was closed before anyone saw it, which surely lessened the impact.
The point remains: if KM says the window was open, and she is an immediate witness, and this is corroborated by others, then the reason the window was open must be investigated.
Thankfully windows and shutters mark very easily, as Prof. Barclay pointed out, so one can gain a clear viewpoint as to who opened it, especially if there are finger prints. Alternatively, because gloves still leave a mark, one could also find out if it was opened by someone who concealed their finger prints. As no evidence was found to suggest this, it is indicative that the window was opened most likely by the person whose prints were upon it.
And in that case, a detective must adapt to reason from the evidence, rather than adapting the evidence to meet a more palatable theory - as indeed the PJ did.