Letters have been placed like this on both sides of the door to show that letters were posted through until too many were left. It also shows that there is now no more room the put letters through as no one has collected them for a long period of time.
This was the writing for when the detective sees 'run if you can' written on the window. We wrote it on a steamed window as it would catch the light behind it which would look effective in the film. It was also a subtle way of the detective knowing that something else was in the house with her.