As for the blood -- my guess is they found a bit of blood on a wall. Probably a small spot of blood. Enough that you might normally miss it if you weren't looking for or at it. My guess is that there's blood spots on our hardwood floors, on a carpet fiber, on a bathroom wall, in the grout of the floor tile that we can't detect. You cut yourself, you hobble to the bathroom and don't notice that tiny bit of blood that fell on the carpet, hardwood, hit the floor or got onto the wall b/c it was too small to really notice. Plus, you prob. weren't looking for it in your efforts to bandage yourself up. Put a special lamp and it's going to turn up.
I'm off to read the article to see if it says how much blood was found. My guess ... a tiny pinprick of a spot.