Lord Grievous said:

Personally I think it would be a little too convenient for Luke's hand/lightsaber to be falling down the same shaft he does, not to mention at about the same time even though there must've been at least a minute between it being cut off and Luke jumping.

Yeah obviously it's the explanation you consider the last because it's so unlikely. But unlikely things do happen. And the thing kinda looks like his hand+saber.

But as I said, one major improvement that could be made is to make it clearer that it is a piece of cloud city like the script says.