IMO the search of 5A was incomplete.
The apartment as a whole was not treated as a potential crime scene (crime, if any, yet to be determined).
A section of the apartment was subjected to a forensic examination that was more in depth than other sections of the apartment.
There does not appear to be any information in the files about how the forsensic search was planned (or not planned, as the case may be), so it is not possible to follow up on why some areas of 5A were seached in more depth than others.
In magic tricks, and other deceptions, the attention (or awareness) of observers is directed away from where the true action is taking place.
This is speculation, but it is possible the window was a 'red herring' to direct attention to the second bedroom, away from other parts of the apartment.
The PJ could have been led to focus their forensic examinations in certain areas, and to not give thought to viewing the whole apartment as a potential crime scene.
This, of course, would imply a level of sophistication on the part of any (potential) perpertrator, that may be unjustified.
The above is not intended to imply any particular theory should have greater weight than any other particular theory.