Looking into Luminol further (thank you Google) I find that simple household bleach can cause Luminol to glow just like it does with blood. If the clinic used bleach to clean the blood and gore from its premises (they were a slaughterhouse, don't forget) the Luminol may glow demonstrating the presence of blood and/or bleach.
They must have a method, because they use this often at trials.
They would be picked to pieces if they couldn't prove it was blood.