content: You point out it's not 'evidence'. Exactly, but it's part of intelligence gathering. Investigators look for things that are 'off', if an area is sensitive or a bit 'off' then they can pry that rock over and have a look.  So basically, if you're trying to cover for something, the cover will upset the normal consistency; you can never cover something up exactly and not over-correct. So you first just look for oddities to tell you that something might be up, then look deeper if you see them.