Self Playing Guitar

I had just put the guitar down on the living room carpet, to get Melissa and I some new drinks. When I came back into the living room, I hadn't thought to worry about the guitar amp still being on, until I heard some quite noises coming from it.

When I got myself back on the floor, I tried to hand Melissa a drink, but noticed that her eyes were wide and staring at the guitar. I turned to see why she was staring at it, and started to hear the sounds of strings being softly plucked. As I looked at the strings, I could see the guitar pick moving up and down. It was strange and scared me more than it should've. I quickly turned the amp of and put the guitar away, not sure what else to do.