Here is a story out of Canada that illustrates the fallacy that every piece of evidence must be explained. The west coast of Canada is a giant crime scene, or is it? For the last ten years Shoes with human feet keep washing up on beaches.  

Authorities have recovered 13 feet in Canada and Washington State.  They have identified five people, two were depressed and mentally ill, and the other three had no apparent history. There is no indication of foul play.

Which came first the chicken or the egg? Are more people committing suicide by jumping off bridges, because of the foot mystery?  Or are more people paying more attention to drifting tennis shoes, in order to gain their fifteen minutes of fame?

The truth is the death and the dismemberment can be totally unrelated.  As a body decomposes and is subjected to the wave action of the Pacific it would not be unusual for the hands and feet to fall off a corpse.  The foot is buoyed up by the shoe and protected which makes it more likely to make it ashore than a hand. Imagination and a penchant for conspiracy take care of the rest.

When police search a crime scene they pick up everything that is there, that cannot be explained. Some of the unexplained stuff could have been brought by the victim, some by the suspect and some by a totally unconnected third party. Some items can be tied to the scene and the victim or perpetrator. Some items will never be put in the context of the victim, suspect or crime itself.