How full are you filling the reservoir? Anything much above the "cold" line will get pushed out the cap when the car gets to full operating temperature.
It sounds like you changed the coolant two weeks ago. It may be that there was still a bit of air in the system, so as that air worked its way out to the reservoir, you appeared to loose a third of a liter of water.
If you are filling it only to the cold line, normal evaporative loss for my four BMWs (two e36 and two e46s currently in the fleet) is about a half cup a every couple of months of daily driving. That is a bit more than my Volvos used to lose. I use BMW coolant and distilled water, and add distilled water between coolant changes.