My things are disappearing!?

I'm 13 and as long as we've lived in our apartment I've noticed weird things. Mostly strange sounds but lately I've been noticing I'll find something somewhere where I'm sure I didn't put it. But today I got a late birthday present from my grandparents in CA. It's a camera. It needs batteries so I put it on my bed, turned around to get some batteries, and when I turned back it was gone! I looked all around my room and I can't find it! It's like it vanished! What should I do?? How do I find it? Help!