I think there might be a ghost in my baby sisters room?

Ok so ive never had experience with a ghost but it was my little sisters bday and she was 3 I was with 3 of my cousins (13, 11, 8) and they told me my cousin Went in to her room to get a hair tie. (we have a big family and with any occasion they all stay over so there family's bags were in my sisters room) and when she walked out the door shut behind her. Of course I didnt believe them but they looked dead serious. Then this morning (I was at my moms house) my dad was in the bathroom, he went into my sisters room to wake her up and left, with out touching anything but her and the crib. My dad went to take a shower and when he cam back out, he heard a bEeping sound. He looked everywhere but finally concluded that it was coming from my sisters room. He went In and found that one of her dollys had fallen onto a little play keyboard (my sister cannot get out of her crib we know that for a fact) I thought well it might of been unstable and just happened to fall. But it would have fallen sometime in the night. He asked everyone in the house (just my mom) and she said she didn't go in there. What do you think?!