Was the staple made of galvanized metal, or of copper? Either one can be toxic, especially galvanized metal. Other possible effects would be for the staple to poke a hole in the digestive tract and cause inflammation/infection in the abdomen.
Metal does show up really well on X-rays. Any chance you could get her to a vet? It's not an emergency, really, although if the staple were still in the crop then they could maybe get it out without surgery. If it's gotten down to the gizzard then it'd be harder to get at. And of course, if the X-ray shows more than one staple, or better yet no staple, then at least you'd know. Because chickens are designed to eat small rocks and carry them around in the gizzard to grind their food, they aren't very good at pooping out small pieces of metal: the body thinks it's just another rock, and keeps it.
Signs of galvanized metal poisoning (it's zinc poisoning) are weakness and loss of appetite, followed by seizures. It can and does kill. The signs can take several days to appear. There is an antidote, although it can be a little hard to get a hold of, and the vet would have to hunt it down for you. I forget what copper poisoning does, but I think it ticks off the liver, which would also cause weakness and loss of appetite. Copper can also be fatal. Sorry to be the bearer of news like this.
I'm really sorry your girl got hold of the staple. They just do things that turn our hair sometimes! Best of luck to you!
