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.
While you make valid points, I am fairly confident that it sure wasn’t Kool-Aid, potatoes or radishes on the hotel/motel blankets! Be real. It’s still disgusting to even think about.