Staghorn Kidney stones can cause hematuria and they are otherwise symptomless for years. Well, other than showing signs of a UTI but no pain until it’s really big. And antibiotics won’t clear you up. They have to be removed via surgery (ESWL is not recommended for large stones… wish my urologist had known that). They don’t always show up when an ultrasound is done. They do show up in x-rays. Staghorns hide from ultrasound because they don’t present as a blob in your kidney but rather as a thin layer coating the entire kidney. It shows on an x-ray because the whole kidney is not supposed to light up, so to speak. Staghorns are most commonly found in women.
But this is a zebra guess. And probably not the hoofbeats you are looking for.