Its night My dog is barking at the window outside but theres no one or anything outside?

You have a choice of:

-Taking the dog out, again, in case he needs to go again.

-Assume he hears something outside which could be: A person, a squirrel, a skunk/opossum/raccoon/cat/other dog, a neighbor talking outside, a neighbor talking in their house that he can hear, some other sound he can hear and doesn't know what it is.

If you have outside lights, turn them on; be sure all doors and windows are locked. If he continues to bark, you could always call the police and ask for a special check of a police car to drive by your house (depending on the area, they may or may not be able to do this).

But, it could easily be another animal, even getting into trash.

You can bet there IS something out there that he either hears, or sees.   Their sight and hearing is far more accute than ours.  

A dogs hearing is a lot more powerful than a humans. Your dog probably hears something outside, that you cannot hear and is reacting to it. It could be an animal or some other noise.

WATCH OUT! Someone's out there about to kill you!

What does "I brought my dog one hour before for a walk." mean?

Could be a critter that you don’t see.