In that case, they cleaned up the blood on the floor, but not particularly thoroughly, and they didn't even bother with the blood on the wall. Instead, they locked the room with the screwdriver... perhaps this killer was in a hurry to get out of the apartment. If they expected that would have to be the case, they might have thought a bone would be easier to discard, unnoticed due to their presence in certain tasks.... though it's too early to say.