Pressure test the cooling system.
I am pretty sure you can get a dye that you can put in your cooling system and I THINK you need a UV light, it will show up the leak in no time at all.
As you should know, if the car is filled up when hot, the water inside is already expanded a bit so in a cold morning the water will come down a bit. Though what you say is a little too much for that
Try looking under your engine oil cap, have a look to see any bubbles, thick milkshake kind of substance. If so, then head gasket is blown.