When my cat passed away, Duchess exhibited the same behavior. There was a chair in my room that my cat use to sleep on. The same night that I had to put down my cat, Duchess started barking at the chair uncontrollably. She was so scared that she would not come in my room. It took her several hours before she calmed down. I knew it was the cat because Duchess and my cat weren't the best of friends. I figured it was Zoey tormenting Duchess one last time before she went over the rainbow bridge.
I was right because the next night Duchess was fine. While I am not religious I do believe in the afterlife. I know for a fact that it was Zoey and I didn't feel scared or threaten in anyway. Zoey may have been the meanest cat on the planet but I knew she wouldn't do anything to hurt us (well she may have gave Duchess one last swap on her butt before moving on.).
