Tiny leaks in the system somewhere. A very minute, pinhole sized leak won't be apparent when the engine is cold; during running, it will escape as steam. It will be so subtle that you won't notice it.
There could be a leak in the heater core. There is also a possibility that the water body could be weeping.
The only way to properly test leaks is to pressurize the system to 0.9 bar (or whatever is written on the radiator cap). An air compressor and a custom radiator cap is required for this purpose, although I just stick the compressor nozzel in the reservoir pipe (not recommended, but it works.). Most shops won't have the slightest clue how to pressure test a system like this.
