You were protecting him as he walked to some place. Probably home. You were invisible and far above him. Just waiting for your chance go protect him. He didn't notice it, but there was a strange person following behind him. They didn't look friendly. As soon as they pulled out their weapon of choice, you flew down and stopped them from using it. Unfortunately, you made too much noise, and due to you currently interacting with another human, you were no longer invisible. His eyes went wide.