Comment Re:This is actually pretty scary (Score 1) 344
Having personally watched a CSI (or SOCO as we called them then) collect samples at a crime scene, I can categorically tell you that at least in the UK, it is standard practice to collect controls as well as specimens.
For example, there is blood around a broken window: the CSI will collect samples of the blood and also swabs from the other side of the room, and from the wall outside. This allows them to exclude various bits of contamination that aren't related to the crime but are present at the crime scene.
Don't let me get in the way of your assumptions though.