If you know what you are doing, all bets are off. You can finger
anyone with kiddie porn and leave no obvious trail behind. All you need is physical access to the computer. Unless the hard drives are encrypted, they are open and vulnerable. And even if they are encrypted, they are still vulnerable if the computer is left running unattended.
This is primarily why it should not be illegal just to possess a certain set of bits and bytes on your machine. You can make it so you can fool the best of forensics experts. And most law enforcement who does the analysis simply use lame-brain software to scan for the kiddie porn files.
It would be easy, for instance, to write a virus that would spread to your machine, download kiddie porn, create fake tracks that would fool forensics, and then delete itself without a trace. Can you imagine if something like that got out and infected millions of computers with kiddie porn?
Well, for one, it would probably end this nonsense of destroying people's lives simply because they had the "wrong" files on their computer!
Not to mention nailing people for files on their computer does NOTHING to stop the production of kiddie porn. As always, law enforcement is focusing on the wrong end of the problem. They should be going after the guys who pervert children in making the kiddie porn. Why don't they do this? Oh, I get it -- too much work. Poor kids. Too much bother for Law Enforcement to go after the REAL perverts. Sorry, kiddies.