Dogs/Barking at nothing?
I have a one year old pomeranian. Today, she was looking at our front door and barking and growling. She continued to do this for at least 30 minutes. I checked and nothing wa at the front door. I showed her nothing was there but she kept barking. Why?
A lot depends on the individual dog and or breed. Some will bark at the wind blowing, while others are more selective. My breed was bred to be watchdogs, and that is a job that they take most seriously.
I have accused mine of being liars a few times over the years, but they probably DO hear something out there that I cannot hear... and that includes noises coming from our garage from the barn cats coming in to feed, and strange noises from the attic/roof, etc. Mice are all trying to get into this house now as winter threatens to arrive, and they can probably hear those nasty things, too, in the walls, etc., or however it is those blasted things manage to get in here.
When I was first out here (in the country on ten acres), I had the dogs all outside on a lovely day that first spring, and they all started barking like crazy. I hollered at them to quit, which they did, but then they started up barking again as soon as I left the window. By the third time, I was quite angry, and I stood there and watched them... all facing the same direction and air-scenting. I looked beyond them, and then a bit further, and I saw smoke rising beyond our barn. It turned out to be a grass fire about two blocks away, and was quickly put out, but those dogs of mine told me about it!
So if barking at nothing is not normal for your little gal, I must assume that she really did hear something that she thought she needed to be alarmed about enough to tell you about. It may not have been much of anything, but many dogs will bark at something they cannot identify. They need to get close enough to sniff it, examine it, to determine if it really is, or isn't, something important enough to be concerned about.
Since she continued to bark after you opened the door, I am thinking that it may also have been something she smelled. Maybe a cat or loose dog walked by your front door, or someone came up close to the house such as a meter reader.
Unless one owns a truly stupid or foolish dog (and there are a few out there), one should always trust his dog when it tries to tell him something. Their senses are so much more finely-tuned than ours, and I have learned to trust my dogs, including when they tell me things about new people we meet.
Next time, check things out a little quicker, and if you find nothing, tell her to be quiet and then distract her with a toy or something in another room and play with her for a bit to get her mind off of whatever alarmed her.