Mary has been in the backrooms before
When Clark tells Mary about the backrooms, she seems freaked out, and the first thing she looks at is her concrete handprint. This is because she might have had memories of the backrooms before. When she goes into the furniture store, she notices wind chimes. They move and make sound despite there being no wind in the basement or the backrooms. In the flashback we got in the movie of Mary when she was a kid, we can hear wind chimes in the background, which is a big part of her memory. She then tries to escape the house through the window. Later, when she walks into the hall with the painting of pirate Clark. It shows pirate Clark lifting a figure up to a window. And at the bottom it says, "Here and now is a two-way street." "What I'm trying to say is that she used to go into the backrooms as a kid, this might have been an escape from her mother as a kid. Also, this explains the weird reaction she had towards Pirate Clark and why she was so slow about getting away from him. She was in shock that this was real, not just her imagination but a real memory. I believe she was friends with pirate Clark when she was a kid since time doesn’t work the same in the backrooms as it does in our world; it's a two-way street, meaning that everything exists at the same time.
I might be wrong about this, but I just thought it was an interesting perspective that I wanted to share