You're right. I remember the stairs from the previous episodes intro animation.
But I also remember some dude's silhouette walking into the frame of the the open door, coming from the room...
Which means
1. Someone is in the locked-now-unlocked room. Which is insane.
It's quite obviously just going to be the MC walking out of that room. The flashforward at the start of Ep 9 is likely a few beats ahead of where Ep 11 finishes off, which means Derek's probably been rushed off to hospital at this point and, for whatever reason, the MC has gone into that room and is now coming out of it.
Not really that complicted to figure out this is what it will end up being.
Now we have to guess it was unlocked since Bella was using it for storage/Christmas decor?
Obvious answer is that Bella used this usually locked room to store some of the furniture that has clearly been movied out of the main room to make way for the massive table. She's been getting very relaxed and carefree as of late, so not unlikely that this led her to forget to lock the door as she normally would have.
But then why would she be freaking out if it gets opened when she'd have to have gone in there to get stuff? Weren't we just helping her decorate in her path?
Because clearly there's something else in that room, something she really wanted to keep hidden, likely because it's traumatic to her. This is where people are getting the idea that it could be a child's room and that Bella lost a young child and is the reason why James left. But even if it's not that, there's clearly something in there she didn't want people to see, and no, it's not going to be something ridiculous like a dead body, or comatose James, or a sex dungeon, it will be something deeply emotionally traumatic to Bella from her past that she's kept hidden but also not removed from that room even though she probably should have.
And why would Maya be confused? Enough for our MC to stop holding Derek to go join her looking at a big dark storage room?
Seems like fabricated conflict and suspense for...towels?
Because again, whatever is in there has surprised her and him. It's a shock revelation about Bella, something they didn't know about her, and that's what leads to clear and obvious surprise. Like I said, a room decorated like a child's bedroom, with toys and other such things, could be the kind of big surprise that would stop her in her tracks and it's likely to be something along those lines.