Depending on how much coolant you are losing, it can only be going one of two places. Either leaking outside or inside. Here's what I would do. Buy or borrow a coolant system pressure tester, and pump the system up to 15 pounds. Let it set for a few minutes, and look for coolant under the vehicle and inside under the dash. If no coolant, pull the spark plugs and turn over the engine. Also check the oil. If the pressure tester drops quickly, you have a major leak and should be easy to find. If it doesn't drop at all, let the motor run till hot and test again.