I agree with everything you say about child pornography.
As for your statement above, Julie Amero would beg to differ, I think.
I work on compromised computers almost every day. Nice computer illiterate folks who believe they have protection because they are using the Norton Antivirus that came with their computer five years ago. They are all at risk of having their computers turned into bots and for storage of files. As you say, "a forensic analysis would hopefully reveal it", "hopefully" being the operative word. In the Amero case it was only figured out because private computer security experts came to her defense. The police and DA insisted, even after they made a deal with her, she was guilty based on horribly inadequate investigation, due to the public outcry. Still she was forced to plea to Disorderly Conduct and forced to forfeit her teaching credentials. If the police dept's forensic computer guy and/or the DA still have a job, it is a travesty.
But moving on, you also might be at risk if you acquired your computer 2nd hand. If anyone other than you has used it. If someone in your household, or a visitor, ever installed file-sharing programs, etc.