Have you ever caught your dog barking at nothing in the backyard?

My little Terrier thinks that she is a guard dog, and whenever people walked down the street, she thought that she should bark at them. I would go to the back door and scold her, but soon, she would be right back barking at something again.
I ended up usually bringing her inside just before the neighborhood kids would be coming home from school, so she was not out there to bark at them.
Then, i was looking on Amazon, and I found a device that is suposed to stop a dog from attacking you, and also works as a training device. It is battery operated, and when you press the button, it gives out an ultrasonic beep that the dog can hear, and people can't.
So, I ordered one, and tried that the next time she barked. I said "quiet!" and pushed the button. Tootsie stopped barking, and put her ears down. Pretty soon she tried another bark, so i repeated the procedure, but holding the button a couple of seconds this time. She flattened down her little ears, turned around and ran to the garage, and didn't bark after that. It worked beautifully ! !
She rarely barks when people walk down the road now, but I keep the beeper in the kitchen window just in case she needs a reminder.