My blood test was normal, but urine showed a bit of glucose. my grandfather had diabetes. could the glucose in urine be caused by reasons other than diabetes, or could it be prediabetes?

Best is to have blood sugar and A1C checked by your MD. Only that can give a clue

If glucose in the urine is positive but blood glucose is normal at the SAME TIME, you may have a congenital kidney disorder which affect the proximal tubule of the nephron. If the urine test was done at a different time, not fasting, the presence of glucose in the urine is the sign of post-prandial hyperglycemia then your HbA1C will be out of range, i.e. you have pre diabetes mellitus or frank DM.