Maybe the angled him just above the elevator shaft. [ She carefully mimes it, holding an invisible human in one hand and an invisible katana in the other, then proceeds to perform the motion of stabbing. ] Blood could have fallen into the shaft or on the sides of the shaft walls, maybe some even got on them.

[ She carefully eases her way back to the trap door. ]

Yeah, we should focus on the shaft or the stairs again. The blood on the victim's desk too.