Mysterious coolant

my 74 slant 6 225 has a mysterious coolant leak. I've topped the radiator off, only to see it disappear with in minutes while the car is warm. there's no coolant in the oil, there's no puddles under the car, no drips, no leaks in the hoses, or heater core. Nothing coming out of the exhausts. It's just disappearing. While I am building a 440 to swap in, I still like to be able to drive the slant while I'm gathering parts but I cannot seem to locate this dang coolant problem. It has to be going somewhere. Could it be leaking into the cylinders and simply burning off? No smoke or haze comes out of the tail pipe though. :violent1: :banghead: