This is not too hard to get provided that you use the detector (press "3" to activate). First, you have to thoroughly clean all the assorted body parts and broken exhibits by moving them to the incinerator, while mopping up the blood carefully. As soon as you've cleaned all that you can see, it's time to use the detector (both green and blue mode) to mop up what's left. The trickiest part is pieces of glass, those damn shuriken which are VERY easy to miss and splashes of blood on walls or columns. Be aware that sometimes these will have almost completely faded to the point of invisibility on certain textures. If you can't see the blood splash just get as close as you can to get the strongest signal on the detector and hit everything around with the mop.