Did you ever see that piece on Dateline where they went into the hotels/motels and sprayed the Luminol (which detects blood splatter)on the mattresses and comforters. So gross. You don’t want to know.
Ewwwwwww
Well, Luminol can detect incredibly slight blood stains - I think about as diluted as a shark could smell, but they don’t have to be fresh. So, even if you bleach out the blood stain completely and it presents no health hazard at all, it’s still going to show up if you spray Luminol on it. And, some kinds of bleach or household cleaners and certain foods (potatoes and radishes among other things, IIRC) show up as well... so what they saw likely was everything from some blood on the mattress from ages ago to the results of a maid using a specific kind of bleach to get out a kool-aid stain. In any case, I’m not worried, if I can’t see the blood or whatever any more it’s way past the point where I could possibly catch anything nasty from it.