...with the door open...

Yet again, jumping ahead of what is actually known for sure. What Mr McCann has said is that they had intended to lock and close the door, and thought they had. When he went back to check he found the door ajar, and assumed that they'd failed to close it properly earlier, and did so when he left (which with a spring lock would be the same as locking it). Subsequently he has come to believe that an abductor had opened the door, and was hiding in the flat while he was there.

Of course all that could be lies - but it is not particularly improbable. I think that most people finding a door open in such circumstances would be likely to think the same as Mr McCann says he did in this version of events. They'd think they'd been a bit careless, and breathe a sigh of relief nothing terrible had happened as a result.

akenaton's assumption that the door was left open by the parents is just that, an assumption.