FREAKING OUT at work - haunted?!

H
So I work at a women's shelter. I'm currently on the night shift, and the only people awake are myself and two residents. 
I'm sitting at my desk in my office with the door open. A blue pen was just tossed/thrown through the door, hit my desk, bounced off and landed on the floor. 
I got up, and the only person out of their room was sitting on the couch watching TV. She had nothing to do with it. 
I'm seriously freaking out. There was only one pen on the desk, and it's still there, so I know it's not that the pen fell. It BOUNCED off my desk. 
My coworker on the shift before me and another resident told me that they heard a loud bang like a door slam on be office side, which is locked and secure. Nobody can get over there. 
I dont know what to do. I'm sitting in my office, heart racing, afraid to go finish my tasks.