I agree with ahmpit ; adocintraining and lornamd-1
Its stephen who actually left the key on da door !

I believe it was Stephen who actually left the key in the door. And I think he was just too absorbed by her presence to realize it. I didn't see anything deeper than a simple oversight.

I thnk that the passion makes u so insane that u cant wait to have sex that u forget to be rational and lock doors and nothng else counts. Ive been in a passionate relationship and u dnt really thnk rationally.

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I totally agree to that.

I thought it was Stephen. It is his subconscious wish to destroy his son's relationship, even if it means destroying both of their lives. It's the Oedipal conflict, with a daughter-figure as the love object instead of dear old Mom as in Freud.

Stephen left the key in the door. This was foreshadowed by his leaving the key in the door the first time he visited the apartment.

Well, Anna had the idea of renting the flat in the first place (I think) and she must have known that there was a big risk of Martyn finding out. She feels compelled to destroy the whole family (at least this is my take on it) not because she wants to but because of her own history. There's lots of stuff going on there before she shows up - recall when Ingrid says that Martyn was the one person for her. And what about Ingrid and her father?

Since the whole thing is shaped by Freudian psychoanalysis (reworking the Oedipal Conflict), you are supposed to see the key in the door as a subconscious wish for Martyn to find out. It is not enough for the illicit lovers to enjoy their clandestine meetings; they must also bring about the destruction of the third point on the triangle. In Freudianism, you not only have the Oedipal Triangle that pits father against son, and the subconscious mind as a source of hidden desires and fears, but also Eros (love and creation) is always wedded to Thanatos (death and destruction).
Trouble is, I can't remember whether it was Anna or Stephen who left the key in the door. At first I thought I remembered it was Stephen. But if you say it was Anna, maybe you are right. The Freudian motivation works for either one of them. What do you think?

I don't see how Anna could have known that Martyn had been called by the landlord and knew about the flat so I would say no to that question.