FaceTime Turned on By itself

So I usually leave my macbook pro closed when I'm about to leave for a long period of time. Today I was dragged off by one of my friends to head out for the night. I left my mac opened with the screen dimmed all the way down. I didn't think much of it and just left it how it was. When I came back a few hours later, it was pitch dark in my room but there was a green light on my mac to show that the camera was on. When I undimmed my mac's screen, I saw that FaceTime was on. I hadn't received any calls or FaceTime requests while I was out so I am kind of freaked out that it was on. And usually when I open FaceTime it will show my face on the camera but it was dark.

So I guess my question is, has this happened to anyone else? And if so, do I need to be worried? I'm running my Anti-Virus scanner just in case.