I have just looked at a previous 5A occupant PA gordon
The doors, windows and shutters were in perfect working order.
There was no sign of the doors, windows and blinds ever having been broken into, but also there was no need to examine [them]. The front door had a double lock that was difficult to lock, but we managed to do it. The patio door did not lock from the outside, only from the inside of the apartment and for that reason when the family left [went out] together we always used the front door."
That is tantamount to admitting it was faulty. for it to be difficult suggests faulty IMO.
http://www.mccannpjfiles.co.uk/PJ/PAUL_GORDON.htm
I dont know whether it is in the files or not, but soon after Madeleine vanished, a couple came forward who had stayed in 5A before The Mccanns. I think they were on honeymoon, but not sure about that now.
They commented that with the door latched (single locked but requiring a key for entrance), the cleaners would just walk in on them.
So they tried double locking the door and the cleaners still just walked in on them. They were not very happy about it.
I cant remember all the details now, but I think that there was something more than plain bad manners on the cleaners behalf ... who should, of course, knocked the door first and waited for an answer. I am not certain about that tho and will happily be corrected if I am wrong on that last bit.