Cooling system and overheating

I drive a 2003 BMW 330i. It started to show a telltale light of radiator and in the morning when I check I found that indeed the level of the water was very low and I refilled. I took it to my mechanic and he did the pressure test as he was suspecting that it might be leaking but nothing. I drove it again but now it started to overheat a little and the followin day I took it back to the mechanic. He took the radiator to the radiator centre for check up and they said it was blocked and they cleaned it. It was mounted back and refilled. I drove again and this time it was not overheating anymore but later the telltale light of the radiator showed. When I checked the following day the level of the water was again very low. The question is what is wrong and where is the water going?