Water

Put cardboard under the vehicle to see if there is any drippings in the AM. Also, get a pump to pressurize the cooling system. Most parts stores will lend them to you. With the engine cold, remove the rad cap, install the hand held pump, and pressurize the system to see if you can see or hear any leaks. If you find nothing, they make a dye to put in the system. Put it in, drive it and then with a black light, check for leaks.