Brief Answer:
Possible causes listed for you sir

Detailed Answer:
Hello sir and welcome.

Thank you for writing to us.

I have gone through your query with diligence and would like you to know that I am here to help. Causes can be many sir, but we need to take into account the fact that the blood found was minute and so the possible causes could be:

+Stones in the kidney or the renal system
+Infection of the kidneys: Should have found increased leukocytes in the blood exam
+Kidney abnormalities (not related to infections) like elevated blood pressure
+Bladder infection: Should have found pus cells in the urine exam
+Prostate enlargement (commonly seen in your age-group sir): Especially considered if you are having other symptoms like poor stream of urine, dribbling of urine, taking time to start urinating while standing, increased frequency of urination at night (nocturia), etc.
+Side effect of certain medications
+Strenuous or rigorous exercise
+Injuries to the renal tract

I would recommend a visit to your doctor for an ultrasound examination of your abdomen and pelvis, and an ultrasound KUB. These should quite suffice in providing us with the way forward or diagnosis.

I hope you find my response both helpful and informative. Please feel free to write back to me for any further clarifications, I am always here to help.

Best wishes.

---