Hi!
You would probably get more advice if you posted this in the "general security" section of the forum. (if you want to move it, you can use the "report" link on the bottom of a post and ask for it to be moved).
As for your problem, it's really difficult to say since you haven't really given much information (and since I'm not an expert on the matter

) - details about your network setup, how many people share the network, firewall software being used, antimalware software being used, what kind of files were open, has he recently been using the files, has this happened before/again,...
But generally, I would say it's a lot more likely that your boss either misclicked something (like opening multiple files by mistake by holding ctrl, which could have happened with a delay - depending on the computer specs) or someone had physical access to the PC. I mean, even if you were to access files remotely from another PC, why would they be open on your bosses PC?