Car consuming anti-freeze coolant faster than normal?

Last week, the anti-freeze coolant in my car was empty. I realized that when the temperature gauge went very high. I noticed some bubbles coming out of the radiator when I opened the hood. After filling up the anti-freeze coolant in the car, the temperature went down. But I have noticed that my car is hogging the anti-freeze coolant very quickly. What could be the reason? Is my radiator leaking? I asked my mechanic, but he said that if it were, we would have seen signs of leakage constantly. But that is not the case. What is your suggestion?