Luminol is a compound that chemifloresces in the presence of blood: It glows blue. Even a trace amount of blood will show up if sprayed with Luminol. Forensic investigators use it at a crime scene to find minute traces that survive scrubbing.
Two days ago I injured my right big toe. I cleaned it, bandaged it, and forgot it. In the shower the bandage must have come off. Then yesterday, while cleaning my bedroom, I banged my toe against the bed frame. Unknown to me, the toe started to bleed again. Oblivious, I moved around the room, changing sheets and dusting, until I happened to glance down and saw blood all over the white carpet. A lot of blood. A very lot of blood. "Yet who would have thought the old toe to have had so much blood it it?" (Apologies to the bard.) My bedroom looked like a homicide had been committed there.
This morning a carpet professional cleaned it all up. But was it really "all?" If the cops ever have cause to perform an investigation in that room, will they find traces of old blood and draw wrong conclusions?
On the other hand, Luminol also chemiflouresces in the presence of horseradish. Maybe they'll just assume someone had a picnic on the floor.