What happens when your dog is yelping and nobody is there and it didn't eat anything or didn't touch anything?

well we were outside and my cousin came in for something and he saw my dog barking at nothing and it was scared it hasn't ate anything to hurt it or it hasn't touched anything. there was no one in the house either.We both came in and saw her yelping and crying for help she was very scared my cousin was right in front of her to. We didn't know if it was a person or a ghost. We are still wondering what should we do to know if it was a person or ghost?