Hoo-ing Spies

I heard the strangest thing! It was a very familiar sound, yet quite foreign. I heard what sounded like more than one owl gently hoo-ing in the trees in the backyard! Yes! Owls in the neighborhood! Either that, or I've got spies communicating like out of a movie with bird calls. =( In all my years here, I've never heard owls in the area, though I know they can be found in the parks or the more forested area around the county. I wish I could determine the type of owl but unfortunately, I am unfamiliar with the different owl birdcalls. The vocalizations did not last particularly long, but I did not hear them flap away either...then again...it's supposed to be quite silent so that they can catch their prey. I didn't see any movement (in the dark...heh...) either. Perhaps they fell silent to watch me for threatening behavior?