"There seems to be an entire city of demons beyond that portal" Leo answers. "I reckon that if they could cross into the world, this place would have long since been a cesspit and we would have witnessed unspeakable horrors. And they didn't seem to notice us across the boundary, either. No, I think they cannot cross it, for some reason. Which only makes whomever has done all those things here even more mysterious."
Leo gives up trying to figure out what was taken, and returns to face the door. "Whomever took things from here had to put them somewhere. This looks like the only place left. Do you have any ideas how to open it? ... Safely?"