Given the amount of plods going through the house, probably looking at all security with little more than cursory glances because they thought it was a suicide and 4 murders, it's unlikely that proper checks were done at the time. It would have been almost impossible for them to recall who checked which window and what state of closure it had been in a month earlier.
confirmed by the Dickinson report the windows and doors were all noted immediately as secured from the inside except the dairy window which was covered by mesh and the upstairs window open a few inches.
We all have our differing opinions and I bow to the judgement which said nothing had to be proven ( meaning it wasn't)- however if I can been defending I would have queried the replies from Barlow and Jones and I would have asked for the jury to visit check the window and also see if that a person of JB size could get through that one window. Just saying that's all.
I am surprised the family did not just find a ladder and say he used that to exit the bedroom window.
I also find it surprising that people who think he was innocent think the silencer could be planted - but that lies could not have been told about this vital bit of evidence.