Did you know the volume of ketones in your urine? Was it slight or high?
Slight to moderate ketones can occur with exercise. Did you work out or do some strenuous physical activity before your appointment?
People on low carbohydrate diets go into ketosis (usually preferably). These are things to consider if your testing does rule out Diabetes.
Plus, this was a one time test. Without other symptoms or clinical presentations to preclude a diagnosis, it may be easier to simply repeat a urine sample next time, too. Shouldn't need to be a fasting sample, either.