There's nothing in the released evidence to show that the police found the "horizontal" catch off its pegs. That would certainly have been reported if true, but all that's known about that catch is that DC Barlow couldn't fully secure it from outside and the police had already checked the window (along with others) and said it was secure. The police simply accepted Julie's suggestion that Jeremy had used the kitchen window as the most likely explanation, despite not finding good evidence to support that. Jeremy had not demonstrated that he could secure any window from outside, and had stated that that couldn't be done.