I was at home last night between 8 and 10 and twice the doorbell rang, but when I went to answer the door nobody was there. Since I had just installed my wireless doorbell and the adhesive holding the button to the brick was not so great, I just took the button back inside to prevent further activities.
Just now the bell rang again. But the button is sitting right next to me.
I assume something nearby is on the same frequency as the doorbell. Otherwise, I will have to start believing in ghosts.