OoooooEEEEEEEoooooooo....

So I was just now sitting on the couch petting Alice - the more social, good-natured of our cats- when she did something totally unprecedented for her. She was deliriously enjoying being petted, when suddenly she stiffened, and actually growled deep in her throat. Not at me; she jumped down, raced around the coffee table, and jumped up on the other end of the couch, to stare fixedly, wide eyed at the front door. I heard nothing at all, but just as a precaution, I reached behind me and locked the door, before getting up to have a look.

There was no bug (and she wouldn't growl at a bug either), and I saw nobody through the peephole. Finally I opened the door- there was nobody there. Alice has settled down, but she's still keeping an eye on he door. I wonder what's going on?