The missing bodies of his companions was, indeed, disconcerting.  His resolve wavered, but he pressed on before he could let the wild theories going through his mind stop his feet.  

As he reached the mezzanine Sohn paused to listen again as he tries to take stock of which door is most likely to lead to the area above the dining room where he heard the 'thunk'.   (Center, if I'm envisioning this right?)

"Find it, Fortuna..." he ordered the dog, letting her check the door to the right since it was closest.  Unless the dog made a fuss here, he'd move on toward the archway.  Behind here should be where the thunk had come from.  His heart beat harder and he took several breaths, trying to mentally prepare himself for whatever he was going to find inside.  

Similar to what he'd done down downstairs, he tried the knob, then took a step away and used his staff to push (or pull) it open.