Why did my dog bark at nothing?

Dogs can hear and smell better than humans can, so its entirely possible that there is something on the air that you simply cannot detect. At the end of the day there shouldn't be a real reason to be really concerned, granted it can be a little freaky, but if I learned anything from my own dog is that sometimes they just get riled up. He may have saw something that got him excited and it left by the time you got out there. Raccoons are crafty buggers.