Did you ever wine and dine in a neighbour's house across the road and leave your kids in the house with the back door open on 4 consecutive nights, despite having acknowledged the obvious distress caused by this act to one or more of them?
They weren't across the road, just a little footpath which they could see over, the shrubs had just been cut … and the group could see the back patio windows. The back patio door was unlocked but appeared the same as all the other back patio doors in that it was closed and curtains drawn. Did anyone else in other flats, ever report having heard someone trying their back patio door? If you can prove that, please do.
Someone had to have been watching them in order to know if the back patio door was unlocked, or come via the front door with a key. The only person that I can think of that was slim, agile and strong enough to enter via the window is Breuckner, but if it were he that climbed through that window, why didn't he leave fibres and finger prints ?
The apartment was almost certainly entered and exited via the front door. CB (not anyone else unless a slim cat burglar or circus performer) might have been able to climb in, but it is my belief that entrance, by whoever, was via the front door with a key. A key supplied by some member of OC's staff. Exit with Madeleine via the front door also, Swap Madeleine over to Tannerman and close the front door using the key alone.
Simples. Occam's razor.
Please don't tell me what they could and couldn't see. I have eaten in that restaurant and weighed it all up.