Yep. Anyone could put anything on your hard drive.
If someone is alone with your computer, it would take less than 5 minutes to create and password protect a directory and unload dozens of kiddie porn images on it.
And your pleas os "I didn't know that was on there;" "No, I don't know the password to that folder;" would fall on deaf ears.
But your files keep all sorts of corrobrating data like dates and sequences of data. Some files keep track of internet sites you access. It wouldn't be hard to verify when these were downloaded. In addition, the ISP has records also to cross reference. The time delay may be due to such investigations and to answer legal questions.
I'm on a university network and I was shocked when I found that I could access several other users computers and even move or delete their files if I wanted. I had just started using Windows XP and had never had this experience with 98. I went in and disabled Sharing from the root of my (C:) Drive. If I remember correctly, one of the user's files was infected with a virus.
If I had wanted I could have deleted everything or even placed files on their hard drive and as far I know, it would have been legal. After all, they essentially gave me "permission" to access their drive.