If the seal on the cap is missing, it allows the water to boil, and expand much more than it should, first thing that the garage should have checked imo.
Just try a new cap, and although this sounds odd, don't worry if the level settles below the minimum mark, the important thing is that you can see some in the bottom of the tank.
My old 2L mondy auto was the same, never used a drop, but water level settled about 1 1/2" BELOW the minimum mark, so I merely used a marker pen and put a new mark on it, it never moved from there, but everytime if topped up to "min" it would chuck it out, and settle where it wanted, even with a new cap.
The caps don't seal 100%, they're set to a certain pressure at which they open, this can and often does occur after switching off as the water heats up much more absorbing heat from the engine, so don't be surprised if you can hear a slight, and I mean slight hiss when you put your ear near the cap after switch off.
tabetha