My daughter has hydronephrosis. No reflux, no obstruction. What can cause this?

Nothing serious. Hydronephrosis can be caused by obstruction or reflux but also can be normal or not a serious problem (usually low grade hydronephrosis). Your doctor has ruled out all the worrisome things (reflux or blockage). Is your daughter potty trained? Young children often hold their urine too long and this can cause mild hydronephrosis. Have her pee every 2 hours and make sure she isn't constipated.
Hydronephrosis. You daughter could have been born with it or has had repeated urinary infections. See a pediatric urologist for help.