Don't rule out a UTI without getting a urinalysis. I've heard of many situations where there were no other signs of a urinary tract infection, and the incontinence had been there for many years, and yet a urinalysis found a low-grade infection that had been hanging around for quite a while.
Once that is ruled out, it is important to continue working with your doctor for a diagnosis. Incontinene is always a symptom of something that isn't working quite right in the body. This kind of incontinence is rather rare in a child your daughter's age, and it's important to figure out what is causing it. The urgency to urinate if felt when the nerves surrounding the bladder start firing before the bladder is truly full. This indicates that there is some problem with the nervous system, such as a problem in the spinal cord, or diabetes.
For your daughter's sake, please help her get help! Hopefully the cause will be minor, and at the end of all your searching you'll at least end up with a treatment to help keep her dry.