I don't believe the apartment was left unlocked at all,
I think that once they realised the 'abduction through window' scenario was too far fetched, that's when the doors became magically unlocked, in order to allow points of entry for the...
creepyman' abductor.
It is possible that the sliding door-window was safely shut and they entered the normal way, when they entered, which likely was rare, unless they needed to. However they used the sliding one to listen, no use to enter.
In this case the open sliding door-window had to be invented when Mr McCann spoke to the PJ on the crime scene.
A check of Mr Oldfield had been added to the second time-line and Mr Oldfield could absolutely not have seen the shutters... He had to enter but had no key and if he pretended he had everybody would know he was lying. Moreover he would have passed the fatal window before opening the door...
The following day, when he started to state, Mr McCann forgot about that detail and said... the truth, that he had entered the flat with his key... Then what could he do when his story came to the point of Mr Oldfield's check ? He couldn't possibly make him listening to the shutters without noting that they were up, so he had to go for the second worst open sliding door-window.