Somebody's Messin With My Mac

I came in this morning and my computer was on. WHAT??????? I know for sure I turned it off last night before I left. Why is my computer on? Of course it was opened to the password screen but it could have easily been restarted. Was Matt messin with my mac? Matt is the IT specialist and he is always doing something. I even had difficulty accessing my blog. What's up with that? What do you think??????