If it is a private computer located on private property under your jurisdiction, and you want difficult to fake evidence, your best bet is to install a discreet security camera.
Also, if the PC is for your use only, there should be nothing stopping you from taking the network attachment with you when you leave it unattended. If your computer is the only one using the internet, you could even take the router / modem, which would prevent all online activity.
Finally, if it's that big of a deal, you could have your hard drive installed into a removable drive caddy that you could simply take with you when you go. There wouldn't be anything left for anybody to get into when you weren't around.