A slide bolted door can only be locked from one side.

If Annie was able to open it, and there were no signs of forced entry… we know it was locked from the outside.

We can rule out Connie because the guard can account for her alibi… and it is physically impossible for anyone to drive from Wits to Pretoria in 15 minutes.

In the case that Luna had committed suicide inside the shed, it is impossible for her to have closed the slide bolt on the opposite side of the door.

Annie must have locked the studio from the outside after attempting to stage Luna’s death as suicide.

And so… we know that Annie is the murderer!