My dog has been spooked for 2 days now, looking up and around for what I thought was a fly, but nothing.?

At first I thought maybe the vent a little loud to diffuse the central AC in the house, but even with it off, he just keeps looking up and around for things. He has not been himself at all. Normal weather, no fireworks, I don't get it. He's only 3, so can't be going senile. Would greatly appreciate any answers.