It seems that they are targeting the sliding balcony doors, as most people now have the internal windows locks and even bars on the ground floor windows. It's important that if your have this type of balcony door, that you get some top and bottom screw in bolts fitted to the middle PVC / wooden section. This stops them prizing the doors open. I also lock the bedroom door where our balcony is when we are out, so that even if someone did get in, they have a much harder job in accessing the rest of the property.