He made a point of saying he'd done it, Lookout, and to prove it, he did it again whilst allegedly knowing he was under surveillance.
Yes, but didn't he leave the window unsecured like he was reinforcing his point that he could get in but not lock the windows from outside?
My thought is that probably the whole family, but June and Nevill especially would know about the kitchen window and would notice if the casement stay at the bottom was left insecure.
So does JB work out another exit that he keeps to himself, for example when he is sneaking a look at the contents of the safe?
I think I saw caroline once say that she had closed a sash catch from outside using a piece of cotton? Seems feasible to me.