The main leftovers of a crime scene investigation are forensic chemicals: luminol is an incredibly useful crime investigation tool, as it glows bright blue when it comes into contact with blood. It can detect blood spatter that’s even too fine to be seen with the naked eye or that’s been partially cleaned up (say, by a perpetrator trying to cover their tracks). Unfortunately, the chemical is like purple spray paint, and sticks to surfaces about as stubbornly. Basically the only thing for it is harsh cleaning chemicals, abrasive pads, and elbow grease – or, failing that, paint to cover. Luminol is especially tough to get out of soft surfaces, which may mean pulling carpets or cutting up furniture.