do you hear any waterfall noises behind the dash when you start up? or when you press on the gas? it should sound like water swishing around. if so, you probably have a leak, or a bhg.
check for that leak first. I had one for the longest time, and there was no coolant on the ground at all. it was just a little drip that burned off in the engine bay, i couldnt find it at all.
take your car to a shop and get them to do a pressure test, that will tell you if you have a leak, and they can find it for you.