I was in the same predicament for a while. I had a fast leak but couldn't find it. I would park the car and watch for leaks etc and nothing, then i would come back and it would have leaked and stuff like that, very frustrating. So i started feeling all around the engine, finding where it could possibly be leaking at all and i found a hose whose clamp needed tightening and viola, no more leak. I have no idea how that small leak let out so much coolant in only one day. So my advice is to check all hoses, heater core (check for wet carpet on inside of vehicle), of course the radiator itself etc. If you've done all of that, i dunno what else you can do. Maybe someone else will...
-E