Happened to me twice once my cousins yorki was spending the night in my room and for no reason she faces a corner in my room started growling and barking and looking back at me then back at the corner.
2nd time zeus was barking mad at night i went out to check what's going on fearing someone broke into the house turned out he was barking at nothing but he was very very upset ears flat, hair on his back and tail up growling showing teeth.
it scared me the 1st time it happened but it doesn't anymore