I had a thought about Ollivander.

It seems to me that there are two major possibilities to explain his disappearance. One is that Voldemort is trying to find out what happened when he and Harry dueled in the cemetary. The other is that Dumbledore is hiding him precisely so that Voldemort cannot find out what happened. The fact that the shop was empty and there was no sign of a struggle makes me think the latter is more likely.