Hi! So I filled up the water by 1.5 liter in the space of a day, which was alarming, so I finally went to the mechanic. It took him a few seconds to find the leak. It was a hose pipe under the air filter. There was a small hole there, and he hooked up a new pipe, so it is all good. I am going to check the car this morning and see how the water level is today.
BTW, I added the FUCHS coolant mixture yesterday. The back of the bottle said that it was already mixed, and to not add more water, but the mechanic was scared and insisted I add water instead of buying more of the same coolant.
If the bottle said I don't need to add more water, then it should be OK right? Anyway, the mechanic was scared so I allowed him to add water.
Also, thanks everyone for your help!